A great source for content, especially images & tools is the Creative Commons licensing website. The search system enables you to find the content you're seeking and also provides a great outlet for developers looking to contribute. Along with these benefits, your use of the system could earn you a link from their website, a highly trafficked and very powerful inbound link.

The Creative Commons partners with many websites, including Flickr, OpenPhoto & FreeMedia to serve media content, and with hundreds of thousands of files in inventory, it's likely you'll find something you like. The only thing they're missing is custom logo design...