Here are some useful digital collections and hints about where to find rights information. Note: While these resources are listed here because they clearly indicate the copyright status of items, some of their collections include copyrighted items. Be sure to check the rights information and follow requirements for reuse.
Image Resources from Research and Cultural Heritage Organizations
The Getty makes available images in the public domain or for which it is the rights holder through its Open Content Program. More information on using items in the Open Content Program can be found here.