The Boston Public Library (BPL) is not just a public library but is also a research institution. The digital resource department regularly adds items from various collections held at BPL to its flickr photo stream. You can view a wide variety of items such as manuscripts from Walt Whitman, old postcards from around New England, 18th century political cartoons, anti-slavery broadsides, and a slew of baseball photographs. Poke around either for fun or if you need images and resources for an assignment. (My personal favorite is a sketch for Make Way for Ducklings).
If you are interested in maps, check out the collection at the Norman B. Leventhal Map Center online.
If you use any images for an assignment, be sure to cite them properly. The library’s citing sources page can help.