"For other images, cite as you would any other Web page, but make sure you're crediting the original creator of the image. Here's an example from Webshots.com, an online photo-sharing site ("brandychloe" is a username):
brandychloe. Great Horned Owl Family. 22 May 2006
If you need copyright free images, use the Creative Commons search on Flickr. Also, use Alternate Sources of Materials to find copyright free images, music and video.