Pretty good for starting off, better than I was lol. I would say try to keep your sigs around 500 x 200 px or smaller, I use 500x200 just because the size just works for me. Otherwise good job, try working around and incorporating various techniques into your sigs, like brushing. Best of Luck!
Also, try and get Photoshop because it has alot more options. I recommend a website called katz
(Watch out for the side advertisements though
. I got Photoshop CS3 (A portable edition) from there and so far it works out wonderful.
Btw, if you want Photoshop websites to kinda start you off go to.
I'm sure this has beginner tutorials.
And for brushes, C4D renders, etc. for either GIMP or Photoshop go to