Where can you find some photos without paying an arm and a leg?  Here are some helpful sites to try for inspiration.

Flickr Creative Commons

StockXchng

FreeFoto.com

Paid Photos

The following sites require some payment, but most images for the web are nominal (less than $1).  The sites also offer some free photos as well.

iStockPhoto.com

DreamsTime.com

Other Free Photo Links

Wikipedia:Public domain image resources

NASA Images

NASA Multimedia

Visible Earch Images from NASA