Microsoft offers thousands of free images, animations, photos and sounds for download. MS has also partnered with stock image resellers Fotolia, iStockphoto, Veer Images, NVTOfficeClips, Presentation Pro, Animation factory and PhotosToGo to offer millions of high resolution, top quality images - but of course, those cost money.
As their website mentions, "Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license." Flickr is a major source of image for the open source community and helps in promoting the unfettered exchange of information over the internet. The WP Plugin called Wp-Inject assists you in searching for images on flickr and then adding them to your articles in a jiffy.
Something to remember is to never use artwork unless you know it is in the public domain. Many popular images such as cartoon characters, logos, comic book characters, pictures of movie stars and signers are copyrighted. This means you could get into legal trouble by using such images without paying for them or getting permission.