Compfight

http://compfight.com/

Search for the type of image you want, and choose Creative Commons license on the left side. 

Check the licence, download the size you want to use, and be sure to copy the HTML for attribution if it's required.

On your start page, switch to Source and paste in the code they give so the photographer receives proper attribution. 

Everystock Photo

http://www.everystockphoto.com/

Select your image, check the license, download and use attribution where requested.

Texture Zoom

http://www.texturezoom.com/

Are you looking for something that's more textural? Try Texture Zoom for free images.

Creative Commons Search

http://search.creativecommons.org/

Wikimedia Commons

http://commons.wikimedia.org/wiki/Main_Page

There are lots of sites where you can get Twitter or Facebook header images. While not all will work on Delphi, either because of size, or because of the way that they only allow uploading directly to your Twitter or Facebook account, many of them you can save the image, then upload into your Delphi header. 

Set Your Profile

http://www.setmyprofile.com

Twitter Coverz

http://www.twitrcoverz.com/

(Unfortunately, this one includes a watermark on their images.)