Have you heard of WikiPaintings.org?
I want to tie in some more Art History with an upcoming 1st grade Tree lesson. I posted on Twitter and Facebook asking for suggestions (aside from Klimt's Tree of Life and some van Gogh paintings I already knew of.) I started searching and WikiPaintings came up. You can search for a keyword and it brings up results from the huge database. The results can be viewed as sort of a slideshow or you can click on them one at a time. I was just thinking the other day that somebody needed to invent this and it has already been done! What a great resource that allows us to search by theme!