ArtStor is a great resource for finding images for art and architecture for assignments and personal use. You must create an account while on campus to access ArtStor. Images from ArtStor cannot be shared freely online, but are good for classroom presentations, papers, and inspiration.
Books are often the best way to find images of artwork. This is particularly true of works of art that are held in private (non-museum) collections. Finding exhibition catalogs or catalog raisonnes of your artist can be especially helpful.
Many museums are digitizing their collections and allow people to use the images in their collections for free.
It is trickier to use Flickr for images because you never know if someone has altered the photograph. However, if you are having a hard time finding an image in one of the more reliable sources mentioned above, it is worth a try.
Your librarian can also help you find images of artwork. Contact Sara (firstname.lastname@example.org) if you need help.