What they're called? Well, he was a painter, but what you mostly find are prints.
I see print of
A Job Well Done on eBay for $10 US, but I don't find any signed originals (or signed prints for that matter) for sale...and not finding folks asking about it either... so in short, there is not a market for them.
... so even if you had one, you'd have a difficult time finding a buyer.
Sorry, I gave it a shot!