The problem with wide header graphics like that is finding suitable images. Anything less than 1600 pixels wide is too small. Most images online are really small and not sufficient for a wide picture. I considered making a montage of several smaller pictures, but after tinkering for a few minutes, I saw that it would look really silly.
I choose the current image because of what Leon mentioned; it's a fortification, and slings are weapons. Misty mountains didn't produce the right affect to me.
I like to think of the "guy" as the slinging.org logo. I found a generic human-shaped outline and then added a sling in his right hand. I think it's very cute.
For fun, here is one from the slinging.org archives. This was the original logo for the site, back when i was first designing it and it wasn't online yet.
A logo I've wanted to try for some time now is like the one above. Imagine a side profile of a guy where the G in SLINGING is. Imagine him firing a sling underarm where it leaves a faded path behind, like the current one, and the pouch is now directly in front of him. If you think about it, it would like like a capital G. Where the horizontal line in the G would be the cords. And the C part would be a faded trail of the rotation. Am I making any sense?
Chris