While their search and finding functions aren't quite as advanced, and sometimes their quality is a bit lower, free clip art and image websites have photos that will fit your budget (at least, they fit mine!). Here's my favorites, with free clip art reviews.
Open Clip Art Library. For free clip art, this resource is the best bar none. It's hard to find things sometimes; try browsing the tags if the search doesn't turn up what you need.
Stock.xchng. A great free photo resource. They don't have as much as some paid stock photo websites, but what they have is good.
Wikimedia. Those pictures on Wikipedia? This is where they are all stored. Check out these if you're looking for topic-specific pictures. Many are rather old (so they're in the public domain), but you'll find great variety and quality here.