What I think I can do..

[Kaden]

The first thing I would do, if I were you, is MAKE THE SIGS SMALLER.

I'm not yelling, just trying to emphasize a point. Take your current sig, for example. The background is alright, fairly interesting, but not interesting enough to justify the 200 pixels, or so of nothing. It also means that you don't need to worry about finding more pictures, or making the font bigger to fill room. Seriously, I think that would make most of your sigs look better.

Another thing that bugs me, personally, is when a pic just cuts off. The clearest example is in the Majin Buu sig, but it appears in several of them. If you're going to use a pic that has a defined edge like that, either put it on one edge, or find a way to work it into the background. It's horribly disrupting to the eye.

Mixing up the colors would be a nice addition as well. You did a good job with the Burter sig, but all of the other ones are pretty monotone. Its fine if you wanna pick blue to work with, but pulling extra colors from the pics that you're going to put in the sig is a nice way to mix things up.

Another thing that, more often than not, makes sigs look way more complete, is to add a border. Just a one pix, black or white border does wonders. It makes it look far more complete than if it just ends. Also, I would suggest pulling down a ton of fonts. Using the same few over and over, regardless of how cool they are (BASE 02, anyone?) gets old.

Those were just the first few comments that sprang to mind. If I think of anything else, I'll reply with it.

EDIT: Added the thing about using other colors.