Bing has an option for finding public domain images. To access this setting go to Bing Images, enter your search term. Upon getting the results, find the drop down box that says "license". Use the "license" menu to select public domain images.
For Google image search, you can do the same. Enter the search term and once the first search has been completed, you can narrow your results by clicking on "search tools." Use the "Usage Rights" dropdown box to select either "not filtered by license" or "labeled for reuse"
Pixabay - Users who do not register can download images, but they do have to enter a captcha code before downloading each picture.
Everystockphoto - a search engine for public domain and Creative Commons licensed pictures
Wellcome Images - Hi-res and Lo-res Creative commons images that you can browse by subject
Open Stock Photography - This search engine for creative commons photos is very useful. Many image results might currently be blocked on campus by the web filter.
Wikimedia - access to the library of commons photos.