It's super easy to add images to your site, but you want to make sure you don' t violate copyright. Failing to attribute the source of your images is just the same as plagiarizing information sources. You don't want to do it!
You often don't need to cite images in the public domain as ownership either doesn't exist, or license to use the work in any way has been given by the owner.
Some copyright owners allow their works to be used if you provide text giving them credit. Some even let you edit their image if you follow their guidelines.
http://worldimages.sjsu.edu/ from San Jose State University
http://sdsu.smugmug.com/ from SDSU