Here are some places you can go to find images for your research. All of them are copyright free.
Image Base- Type a search in the box or browse the galleries, including City, Nature, People, and Powerpoint Templates
LIFE Photo Archive- Hosted by Google. Search historic images by decade.
Pexels- High quality stock photos
Pics4Learning- "a curated image library that is safe and free for education"
Pixabay- Free images and videos, all copyright-free through Creative Commons CC0.
The Morgue File- "Free images for creatives, by creatives"
McKinley Library - 121 East 3rd St, New York, NY, 10009 - 212-674-3180; ext. 3071 - firstname.lastname@example.org