I've got this hub bookmarked:
Do you have image editing software? My main image site is Dreamstime.com. They have a big selection of free images but they will typically need to be resampled to a web format.
The premium photos can be downloaded in a web-friendly format and the prices are good.
You can find hundreds of premium pics for under a dollar. Each pic has a different price so shop carefully. (I'm not an affiliate. I just use this site a LOT.)