Anyways, here's some real tips.
Try to aim for a more banner like shape. This means a long, horizontal rectangle and not a square. It fits better, and is more of a traditional web banner. Secondly, Trans is your friend. Either trans it, or give them borders. Also, you should include some original artwork into it, not just getting text and spamming filters on it. I've found putting a dropshadow behind text never hurts, either.
Ask for some personal touches the clan might want to add. I did my clan's personal touch by Bloxel Rendering some of our member's builds into a 2D graphic. Turned out nice.