I assume that, like the Templars, the Hospitallers would have borne the same arms on their shield as on their banner. David Nicolle has also assumed this, in his Osprey book Knight Hospitaller, 1306-1565.

The earliest pictures of Hospitaller shields that I have found are from Guillaume Caoursin's account of the siege of Rhodes in 1480 -- these do show a red field with a white cross. This is not conclusive, but it does seem most likely that the Hospitallers' shield had not changed, and had always been red with a white cross. Caoursin's illuminated account of the 1480 siege of Rhodes survives only in manuscript, so far as I know: Paris, Bibliothèque nationale, MS Lat. 6067. I am not aware of any modern facsimile editions. There are 15th and 16th-century printed editions, but they are illustrated only with black-and-white woodcuts.

Individual illuminations from the manuscript have been reproduced in various places. Fols 40v and 175v, which show the shields, are reproduced in Elias Kollias, The Knights of Rhodes: The Palace and the City (Athens: Ekdotike Athenon S.A., 1991), ISBN 960-213-242-6, pp. 50, 53.

Alas, I do not know of any Hospitaller frescoes like the Templar frescoes at S. Bevignate in Perugia. Matthew Paris's Historia Anglorum has one image of the Hospitallers' banner thrown down in defeat in 1239: this shows a red field with a white cross (London: British Library, MS Royal 14 C vii, fol. 130v). Again, in one manuscript of his Chronica Majora, Matthew showed the banners of the Hospitallers, the Templars and the king of France: and again the Hospitallers' banner has a red field with a white cross (Cambridge: Corpus Christi College, Parker Library, MS 16 fol. 141). He never shows the Hospitallers' shields. However, as I wrote above, I think that it would be most likely that the Hospitallers' shields matched their banner. Reproductions of both Matthew Paris's drawings of the Hospitallers' banner appear in my The Knights Templar: A New History (2001), pp. 64, 166.