Free Webpage Headers – A very nice collection of hundreds of 800 x 200 website headers which can easily be used for Blogger. Images are organized by category which makes it much easier to find what you’re looking for.
Dreamstime – This is actually a pay for image site but they have a free images section you can benefit from. Click on the “Free Images” tab once you get to their home page.
stock.xchng – By looking at the name of their site you wouldn’t think it was any good but looks can be deceiving. They’ve got thousands of free images that would work for your blog header or for inserting them into a blog post.
Stock Vault – This free image site is dubbed the stock photo sharing website where designers and photographers can meet to share their images. Decent selection and more targeted towards designers but you might be able to find some goodies for your blog.
FreeFoto – This last one is a bonus since the list was only supposed to be 5. It’s a pretty good site where you can find all sorts of images for your blog header, etc.
45 Free Beautiful Blog Headers – This is a great list compiled by Smashing Magazine. Some of the headers include the .psd source files so you can really customize the header if you’ve got Photoshop experience.