In contrary to what the person above me said, It's fine to use GIMP, of course It's not as GOOD as photoshop, but some of the basics are almost the same, though sooner or later you'd have to switch to photoshop after exploring GIMP, I believe you'll do better, and have a smoother knowledge about images after that.
About your signature, I believe you just slapped an image over a gradient, lowered the opacity and added your text, which is NOT bad. It's all about exploring, and imagining. You can look up a few tutorials for gimp around the interwebs. The best way to improve is by reading these tutorials and incorporating your own style with it. Good luck on your further ones. That's a good way to start.

