Call them headers, covers or toppers, you need to know how to create great first impressions for your Zeta style forum.
How big should your image be? Rusty discusses it in the ZetaBeta forum.
If you're trying to design a photo off of Delphi that will work in the header, the approximate dimensions are 1200x250. You should be able to upload images that are a bit larger or a bit smaller and crop them to the correct aspect ratio using the tools in the Control Center. An image smaller than 1200 will look a bit pixelated as it gets stretched. You probably should avoid super huge photos because they'll slow down the page load time for your members.
As long as it's a high quality image, it doesn't have to be the exact size of the Zeta header. Here are four versions of the same image, all showing approximately the same portion of the image:
You can see that the small version is pixellated, and really doesn't look very good.
Medium isn't bad, but the center of the flower and the bees lack sharpness.
Large looks about right.
Does the extra large look any better? It might not be worth the extra download time.