Check the license regarding use and attribution of images from these sites
Creative Commons enables the sharing and use of creativity and knowledge through free legal tools
Wikimedia commons is a repository making available public domain and freely-licensed educational media content (images, sound and video clips) to everyone
Flickr enables people to store, sort, search and share photos online
Morguefile free high resolution stock photos site with an intriguing name
Free Images is a site where creative people exchange their photos for inspiration or work
everystockphoto is a license-specific photo search engine, currently indexing and searching millions of freely licensed photos
Photopin helps bloggers find photos for their blog and makes adding them to their post fast and easy.
Alegri Photos is a collection of Public domain and Creative Commons images
Offering work under a Creative Commons licence allows more liberal use of material, but only under certain conditions. Look for these symbols on Creative Commons images. Here's more information on licences, if you require it.