Ok, I hope this question isn't too off-topic...
I was looking through the X10R3 source tree trying to find the
earliest paint program for X. I wasn't able to see anything that
looked like a paint program.
Xpaint might be the oldest, wikipedia says the first version appeared in 1989.
Searching for xpaint on tuhs returned no matches, but I saw that
4.3BSD-Tahoe had some old X programs but nothing listed there seemed
to be a paint program.
Maybe xgedit? It's listed as a "simple graphic editor for the X window
system", but I don't know if it really qualifies as a paint program.
Mark