Legally you cannot use a photo you find on the web with out the photographer's permission.
Stock photo websites allow webmasters, marketers and publishers to locate pictures for their marketing and promotional materials without the hassle of organizing a photo shoot. With copyright laws businesses must be very careful in using unlicensed photos.
My favorite stock photo website is http://www.photo-wizard.net
Here is where you can find some other inexpensive stock photo directories:
Another site list can be found here: