I would suggest tags:
hills (or foothills) for the paintings that have distinct foothills
clouds (so that we can differentiate paintings according to what the sky looks like)
When I tried to tag paintings it just gave me random paintings to tag, can I choose which painting to tag?
And maybe it would be easier if tags were split into groups (for instance - first general tags, like winter/desert/seascape/forest scene, then specifics, like river/lake/waterfall or evergreens/trees)
Great idea, I'm happy that you're always trying to improve the webpage. Thanks 🙂