As you are probably aware, most image vendors restrict you to the number of reproductions you can make using their images. Many have additional licenses that you can purchase, but they are often exorbitantly priced (specially if you need to use many images-like myself). They also generally have some form of size restriction such as don't use images over 72dpi.
Where can I source images for ebooks that are not restricted by reproduction limits nor restricted to 72 dpi?
I don't mind paying for images as long as the prices aren't exorbitant.
MY OWN RESEARCH
Flikr allows you to search specifically for content that is ok for commercial use and is licensed under the creative commons licence. http://www.flickr.com/search/advanced/?