WSU Libraries has a couple of databases that allow you to search art objects and find high-quality images. They are:
Two other ways to find images include Google Image Search and searching in print books in the library. Google Image Search is great because you get back a LOT of results, so you're pretty guaranteed to find something; but, it will require you to sift through your results and determine if something is credible or not. You might find something on a random blog, but you don't know where they got it. You will need to make sure that they provide full and correct citation information. The library databases above will do that for you! But Google Image Search might be a good place to start.
You can also search in books in the library (type in your search and add "art" or "painting" or whatever you are looking for, and select Ebooks under Type). I've given a couple of electronic book examples below to sift through!.