My thought was it was Big Henry.....but I don't know that he put much forethought into his paintings. I vote for mouth. I also think, though it's maybe been said, that paintings weren't produced necessarily for specific books. I believe there was a big batch of "portraits" that Martin had that ended up in Ham to War All The Time to Hot Water....but that's just a guess. I wonder how much time he spent on each small painting. 10 minutes?
I know some of them look like they were made in 10 minutes, but I don't think most of them were done that quickly. He said (somewhere) that entire days were spent making paintings and complained that he couldn't get any writing done on those days.