Anything but this horrid magic tree... I'd recommend something simple. Since we have the background of each posts as a solid color, we don't need an elaborate picture that won't even be seen. I did like the idea of the stars we had before (I think we had something like that).
I'd also stick with the Purple/Silver color scheme as that's the clan colors. But that's more of an overall appearance item as opposed to background.
Looking at your posts in this topic group, you're looking for ideas on the look of the forums, all need to be closely related to each other. A great looking logo/banner can be made but clash with the background if created without the thought of the other in mind.