it is kinda neat that they are somewhat sparse (although i wouldn't go as far as to say ZEN). Because they are sparse and don't catch your eye they could perhaps blend it with the background of the forum - assuming the colours match.
One other thing about the size. I visit flight sims forums where people have sigs liek this and when they all repsond to the same thread 5-10 times, it takes forever to load and the threads are so long to scroll because everyone has a sig liek this.
Not bad, but the filesizes definitely need to be smaller. Besides physically shrinking them, you can get more JPEG compression out of those before they start looking overcompressed.
