Call them headers, covers or toppers, you need to know how to create great first impressions for your Zeta style forum.
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.
Select your image, check the license, download and use attribution where requested.
Are you looking for something that's more textural? Try Texture Zoom for free images.
Creative Commons Search
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