The most famous and influential visual portrayal is from the illuminated Froissart manuscript (c. 1460–1480), featuring ‘Jehan Balle’ and the rebels with the banner of St George (British Library MS Royal 18 E. I f.165v).

Nineteenth- and twentieth-century novels allowed for much more description. Ball has been presented regularly with piercing, fiery eyes and hair circling his tonsure. His physical appearance differs. He can be bearded, clean shaven, muscular, lean, or well fed. His appaearance changes with the fashions of the time, as we might expect: in the 1970s, for instance, Ball’s hair grows longer and somewhat wilder. Often, physical descriptions are brought into the service of portraying Ball in certain masculine constructions, such as a menacing threat from the lower orders, as a strong leader, as handy in a fight, or as a stoical, pious, wise, mystical, ascetic (etc.) religious figure. There are practical reasons too for decisions made. The lean version of Ball recurs because of a standard feature of the narrative of the revolt: Ball had been imprisoned. The more substantial versions of Ball may be influenced by Friar Tuck of the Robin Hood legend, or designed to produce a threatening figure who could physically intimidate the ruling classes. Here’s an example from the Daily Worker (19 July 1938) of a Ball (accompanying a Hymie Fagan article) who is not to be messed with:

From around the same time (and with different political connotations) is a physically quite different Ball from Bernard Fleetwood-Walker’s John Ball and the Peasants’ Rising of 1381 (1938), a mural for the Council Chamber at Essex Country Hall, Chelmsford, where it still hangs today next to a portrait of the Queen:

One of the curious recurring features of the reception of Ball is that he is regularly presented as having a large nose. William Morris’s A Dream of John Ball was no doubt as influential as ever here (‘big…clear cut’ nose with ‘wide nostrils’) but this may also have been a standard description by the time Morris was writing.
An influential source was a copy of the illustration in the Froissart manuscript in Henry Noel Humphreys, Illuminated Illustrations of Froissart: Selected from the MS in the Bibliothèque Royale in Paris and Other Sources (London: William Smith, 1845), 73–74. The famous image is copied but with a distinctively Victorian hue:

Today, portraits of Ball continue and perhaps the most common (after the Froissart tradition) is from Red Saunders’ photographic project HIDDEN which recreates a radical tradition with a focus on realism, including this picture: John Ball (Hedgerow Priest) 1338–1381. The processes involved in capturing the images is also freely available:
There are plenty of visual portrayals of Ball on postcards, in film, in magazines, on the stage, in photographic art, in illustrations, etc. and the history of this tradition is in need of further research. It would make an ideal PhD project.