For some web sites I want to have a header text on top of an image. I don't want the image to be a photo, a picture of anything nor a repeating textured background. I am not sure what I am looking for but the image needs to be subtle and not strong. An example could be random color waves. The idea is to use these images instead of a boring solid color as the background image in a header div.
Any free sites which provide such images to borrow from?
It's not quite free but it's pretty cheap... http://www.istockphoto.com
They have a wide variety of artwork (not just photos). It sounds like you're looking for an illustration and not so much a photo. I searched for "gradient" and found this.
Kinda cheap, but for something really subtle and not too fancy you can use jQuery UI's ThemeRoller to configure the content area and use the background image that it generates. This works for me sometimes since I'm a developer and have zero graphic design skills.