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Merely one tyrant eliminating another by means of execution: that was Robespierre's dismissive judgement on the death of Charles I at the will of Cromwell. He refused to see any parallels with the impending execution of Louis XVI at the hands of the noble, democratic French Revolutionaries. Louis XVI himself, on the other hand, had always been presciently fascinated by the character of Charles I. As a small boy at Versailles he had recited a prepared speech to the historian David Hume; later, Hume's account of the unfortunate British monarch became one of his favourite studies and, in preparing himself to die in January 1793, he took as his model Charles I's conduct on the scaffold nearly 150 years previously.

At the beginning of the 19th century, however, William Hazlitt reported a conversation among his friends in which all agreed that OliverCromwell "with his fine, frank, rough, pimply face, and wily policy" was the only statesman in history they would wish to have seen. Towards its end, Sir Richard Tangye, a rich industrialist, admired Cromwell so much on grounds of his religious principles and his integrity that he dedicated much of his wealth to forming a private Cromwell museum, packed with books, pictures and artefacts, at his estate in Cornwall.

All these attitudes indicate how far the reputations of both Charles I and Cromwell have always been adapted to suit the purposes of the time - or the individual. Long before there were spin doctors, there were propaganda warriors: reds (the royal colour), greens (the Leveller colour), and infinite shades in between. Now, by historical coincidence, the 350th anniversary of Charles I's execution (30 January) is found to occur within a few months of the 400th anniversary of Cromwell's birth (25 April). Two new exhibitions, the Queen's Gallery with Charles I: King and Martyr, and the Museum of London with Cromwell: Warts and All commemorate the respective events. And, in a sense, the propaganda war is continued.

The position of the Queen's Gallery is significant: it is tucked in beside Buckingham Palace and you reach it by what I always think of as a servants' entrance - although "subjects" may be a more appropriate word. The interior, however, houses a rich jewel of an exhibition which no one interested in the connection of art to politics should miss. It is also a fascinating exposition of the uses of propaganda. (You can't help noticing, in this connection, that the official copyright line on the excellent catalogue by Jane Roberts, Keeper of Prints and Drawings at Windsor, is "Royal Collection Enterprises Ltd".)

The position of the Cromwell exhibition at the Museum of London is also significant. London, as the visitor quickly learns at the entrance, was always heavily - and influentially - in favour of the parliamentary side in the Civil War. We are encouraged to think of Cromwell as London's local hero (no doubt planned events at Huntingdon, his birthplace, and Cambridge, which he represented as MP, will stress their own connections). This is an excellent, well-planned exhibition in which a great deal is interestingly displayed in a small space, having as its kernel that Cornish Tangye collection, handed over to the Museum of London in 1946. It does not have - how could it? - the sheer quality of the pictures and objects from the Royal Collection, such as Van Dyck's relatively small version of his equestrian portrait of Charles I, let alone the famous Van Dyck triple heads which alone make the Queen's Gallery worth a visit. Yet there are objects of great interest, such as the music manuscript of Cromwell's cousin Anne - music she played to him herself.

Cromwell: Warts and All does have its bull point, and this too is announced to the visitor. Cromwell has been more written about than any of our other rulers. We are at least half-way towards Sir Richard Tangye's conviction that "the nation" owed more to him than to any of its other leaders, even if the tactful subtitle "Warts and All" does presumably allude to two nations who may not feel the same way - Ireland and Scotland, against which he led military expeditions.

What the two exhibitions, taken together, reveal is the need that was felt, in the case of both men, to build them up and project a suitable image both royal and strong. In the case of Charles I, hereditary King, it might be thought that his royal birth alone was sufficient for at least the first part of the exercise. The death of the gorgeous, brilliant Henry, Prince of Wales in 1612 left the 10-year-old Charles, Duke of York, very short for his age and a stammerer, as the heir. None of this deterred the image-makers: in many cases, engravings of the late Prince were simply made over to do for the next one, and the trappings of the title were left in, although there was a four-year gap before Charles's creation as Prince of Wales, presumably for mourning's sake. Charles I's height could not even be guessed at from the pictures at the Queen's Gallery.

Charles's marriage to Henrietta Maria, Princess of France, shortly after his accession in 1625, involved new image-making. Henrietta Maria was tiny, very dark, with terrible teeth. In the royal pictures she emerges as exquisite, almost as lustrous as the great pearls round her neck and at her ears. She also provided, very quickly, children. Here, as in Hendrick Pot's charming study of Charles I, Henrietta Maria and Charles, Prince of Wales of 1632, we find the first pictures celebrating what George VI would call "the family firm". (The images of Henry VIII and his children, with the odd wife portrayed as a rather small figure, were always more about the King's potency, the survival quality of his dynasty, than about a family as such.)

The third stage in the development of Charles I's image was based on strength; as his political troubles developed and a military solution became increasingly likely, the mighty leader succeeded the tender father. This, of course, was the first stage of Cromwell's image, since his power derived from his generalship in the Civil War. William Dobson's Charles I in armour, in the Queen's Gallery, was painted during the early Civil War period, while the first images of Cromwell show the "rough, pimply" soldier's face of Hazlitt's imagination. For a moment, then, it suits both royalists and parliamentarians to denote their leaders as military men.

But then Cromwell's crossover begins. Moving from General to Lord Protector in 1653, he proved no more capable than Charles I of providing that stable, parliament-based government for which the war had presumably been fought. Increasingly, Cromwell's inclination moved towards the notion of restoring kingly government, on the grounds that the people knew their duty to the king and he knew his to them. In short, a protector had an unnatural base of power, and a king had a natural one. By 1657, eight years after the execution of Charles I, and the formal abolition of the monarchy, rumours that Cromwell would ascend the throne as Oliver I were rife, and the evidence is that he did seriously contemplate it, until some of his old friends from the Army persuaded him either that it was against the will of God, or that the Army would not wear it, or both. Eighteen months later Cromwell was dead, so we can never know whether he would have changed his mind.

Cromwell's royal transformation act, so surprising to those who think of him only as the Arch-Republican, is illustrated at both exhibitions. One engraving, by Peter Lombart, sometimes known as The Headless Horseman (1655) owing to its Vicar of Bray-like changes of subject, shows Cromwell in regal pose on a kingly horse with sword, sash and attendant curly-headed page or squire. The same pose - in fact, the same engraving - was subsequently used to represent Charles I posthumously, and Charles II, Cardinal Mazarin and Louis XIV. In real life Cromwell made the protectoral style as close to the royal as he could - using many of the art treasures of Charles I at Hampton Court.

Ironically enough, in death it was Cromwell who was buried with all the pomp of a king, as pictures and a surviving hatchment show. The best that the supporters of Charles I could do, denied any kind of state funeral in 1649, was to turn him into a martyr who had died for the Anglican religion. An engraving by Wenceslaus Hollar shows Charles I with his own crown in the dust and a heavenly crown approaching him.

What is it about monarchy? After the Restoration, Cromwell's body was dug up, and ceremoniously executed in its turn. Although plenty of people in England today are descended from Cromwell, there are none who bear his name. When the last descendant in the male line, one Olivaria Cromwell, applied to George III to preserve her name on marriage, the King was heard to mutter: "No, no more Cromwells." Royal Collection Enterprises Ltd, on the other hand, is symbolically flourishing.