Hrm... nice start for someone new to graphic work. However, and this should not be taken as flaming, but constructive criticism... please take note of this. I am NOT flaming your work, just guiding you, helping... -watches back carefully-
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You have entered the Burning Text Plains...
'Shaden, the Graphics Guru' has attacked...
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The signatures are... full of bad qualities. That sounds harsh and critical, but it is true and I can't think of a nicer way to say it. I suggest you practice your layering a lot. For example, the light on your new signature should probably be on an under-layer from the render with some sort of layer effect to distort it from being so sharp. I'd go further into detail but you can find a million tuts on Deviant Art and JaKool. (both are excellent resources for tutorials on PSP/Photoshop and you can probably convert them into GIMP language with certain limits.)
Next, practice text a TON. Your text is plain, stands out way to hard (detracting from the render itself), and could be softened in a few cases. Text is honestly the hardest thing for me and many graphic designers in general, it takes forever to master imo. But really, practice it a lot by using layer effects, brushes, opacity, and layering. Text can kill a signature, it really can. So you want to take a lot of time on it.
Also, try downloading some brush packs. I'd look for Abstract and Grunge, possibly Tech but that gets a little advanced. Figure out how to use these brushes on the foreground with duped layers and effects - you can really create some amazing things with a simple duped layer and blur'ing the bottom one. Brushes like abstracts and grunges can really fill up empty space.
C4D's and Fractals are also amazing things to begin using. C4D's are made with Cinema 4D which is a commercial program, and Fractals can be made with Apophsys (a free program) which I sugest trying if you want to really get indepth with your signatures. Those are completly optional though and many can be found at render sites for free.
Last tip, practice making borders (even a simple black 1p line) around the signatures exposed areas. It will make it look better in general, imo. If it seems to sharp, lower its opacity until it looks right (or use effects on the layer). Remember, a good signature can take a few days to make. Some may take hours and still look great, but never try to push 4-5 hours a signature. If it takes 2 days, it takes 2 days.
Please, do not take this as a flame. Graphic design is hard and mastering it takes forever, and no, I'm not some GFX Guru, (that was a joke) I've been using the programs for a while and seen many signatures, so I hope you take my advice the next time you open up gimp. Mainly because you have the right idea, but you do need a lot of work. Again, I'm not a GFX Guru, don't flame meh.
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All in all, great job man and keep up the hard work!
EDIT: Do not save and host these things as .JPEG's - it screws them up horribly. Use .PNG, it looks a lot sharper!