Professors have mixed feelings about Wikipedia. It's a wonderful resource, but the changeable and potentially incorrect nature of the information makes it a source we hesitate to let students cite.
Without getting into this debate just now, I'd like to point out that Wikipedia offers a surprisingly good selection of useful images, which are by definition freely available for download.
While Wikipedia is unlikely to replace other resources for images of paintings, it can be great for photos of sculpture and buildings. This morning, for example, I was able to get several hi-res photos of works by Czech cubist sculptor Otto Gutfreund, and also to get hi-res photos of buildings designed by the Czech modernist architect Josef Gočár. These are original photos taken onsite by Wikipedia contributors, not scans from books. They come with photo date and various other information about the shot, so that in your PowerPoint presentation you can contrast your scan of a 1920s photo of Gočár's Legiobanka with the Wikipedia photo Petr Vilgus took in 2006.
Wikipedia is, of course, always looking for contributions. Consider photographing significant buildings or monuments in your town and uploading them to the page for that architect. You can find out about Wikipedia's photo policies here, useful whether you are uploading or just downloading.