Practically all painting or drawing includes some sort of symbolism. Sometimes it’s so obvious that we don’t recognise it as symbolism (picture of cow = cow), but other things aren’t (spiral = ?).
I read about an American professor that was visiting the various cathedrals in France and started noticing that the stained glass windows were interweaving the parable of the good samaritan with the story of the garden of eden in a fairly consistent manner. He didn't understand why they were doing that until he found records of medieval sermons explaining why they were combining these two stories. It was a symbolic message that would have made sense to medieval peoples, but had been lost over the intervening centuries.
I think it's a trivial observation. For example, it's clear illiterate people in Europe still knew what a cross meant when on a building, there is no doubt about that. There are many more religious symbols that were also well known. Flags and official seals similarly had well known meanings in their own areas, as did various military symbols.
OK, but symbol literacy, at least in this sense, is a long way from literacy in a language having the grammar of the spoken language of these people (or a grammar equally as expressive, as we see, for example, in the case of scholars who read and wrote in Latin while conversing in the vernacular of their time and place.)
Interesting - where is that from?