In Praying directly before an aron the question is asked whether it is proper to pray in front of a Paroches that is decorated. I am aware of a few Shuls that do not have any decorations or words on the Paroches. I never thought about it, yet now think that most likely they do this, to avoid the issue of praying in front of pictures or drawings. Unable to find a source from previous generations discussing this phenomenon, makes me wonder when in history did Shuls start having a decorated Paroches?
There is a long, long history of decorated curtains/parochets in front of arons/arks.
The first one I found in Tanach was in the original Mishkan, according to Shemot/Exodus 26:31 - 33, dividing the original Ark's "most holy" area from the just plain "holy" area in front of it. It was multicolored and had cherubim on it.
The second one mentioned was the one in the First Temple, briefly described in Divrei HaYamim 2/II Chronicles 3:14 as also being multicolored and having cherubim on it.
The third ones I could find mention of are the veils that were in Herod's Temple, as described in Josephus' Jewish War Book 5 Chapter 5. Oddly enough, he only says there was "a veil" separating the middle part from the Holy of Holies, but he DOES do a good job describing the extremely well-wrought "Babylonian" curtain separating the first room of the Temple from the second.
I wish I could accurately answer the question in regards to decorated curtains in Synagogues, but I could not find any information on them in any of my archaeological literature. They have found Torah ark niches and lintels that once were part of the arks themselves in various excavations, but no trace of the curtains themselves - woven materials decompose a lot faster than stone or floor mosaics. The synagogues' decorative stones and mosaics often have images of Temple implements on them, so it seems logical to me that our forebears would decorate their Torah Ark curtains, in imitation of the Temples' and Mishkan's, and they're most likely a very old tradition.