They are good for a begginner.
Here are some tips.
*Try to stop using whole characters as much, and just focus on the upper bodys.(unless you are making a sprite tag)
*Also only use 1 or 2 renders and maximum in a signature. This makes sure the signature has a focus point rather than just confusing the eye.
* Try and cut out better. It can take a while if the color select doesn't take out all of the renders bg color, but its worth it.
* Try using c4d's or abstract art, this will give depth to your background.
* Do a bit of blurring or smugging on your renders to give them more affect.
* Place the render in the center of the sig, or just to the left or right of the center. ( You don't always have to do this)
* Try Finding better renders. Try this site
http://www.planetrenders.net/
as renders are going to be the center point of your sig, they have to look as good as possible.
Hope this helps. If you are still stuck pm or pager me and ill help you some more.