There's just way too much 'stuff' in that picture.
Take a look at the banners in each of the existing themes. First of all, the words 'ODDWORLD FORUMS' must be the main part of the image. People shouldn't need to search to find the name of the site. Secondly, each banner only has one or two main picture elements. For instance, in Industrial Theme there is a slig on the left and a snoozer on the right. There are two other pictures (Splinterz and a glukkon), but they're very much in the background and don't stand out. There is a clear geometric seperation between the slig and glukkon on the left and the region of the picture showing Splinterz.
All of these elements combine to make it an attractive, but unobtrusive banner.
Also, you haven't considered where the links above and below the banner will be placed.