I think its very simple and i have made alot of customizations any ideas for improvements.You need a better skin btw.
Thanks fo that you've reminded me to sort all that out.You need to get GD installed if you are going to use Image Verification on sign up.
I think the register image would look better if you centered it mate,
I reckon sherwood has got a point about centering the register image (I would personally do it a bit smaller too), and I too think the custom images will make a difference, and about the favicon, try http://www.freefavicon.com/, you can use any (square) image and do rolling text if you want and they will make it a downloadable .ico for you.
I liked the simple theme, just fill it with the posts and it will look quite good