Be sure to give proper credit and check out the details on each photo just to be sure...
Flickr - you can do an "advanced search" here for photos under "Creative Commons License". If you find something you like, you can use it for a background or header on your blog, provided you give credit where credit is due. (read the details of the license for the particular photo, as some don't want any modifications made).
http://www.sxc.hu/ Stock exchange has lots of cool photos waiting to be used (as well as some that you have to pay for).
A couple of samples (I'll share blogger coding for backgrounds in a later post):
Here's an example of a portion of a photo used as a background. Appropriate place to give attribution would be in the sidebar - generally at the bottom (as shown in the example).
For this blog, I decided I wanted a Roman motif repeated in the background. I followed the flickr instructions above searching for things like "Roman", "Roman wall", etc. before finding something that I liked. (I think the end product might not be authentically Roman, but it works anyway). I did have to get special permission to modify the photo (I was using only a small portion of the photo). The photographer was actually quite pleased to give permission - it was a quick e-mail answered the same day.
Please share any of your favorite copyright free photo or clip-art sites.