The very first two things i notice are that the signatures seem really low quality. Maybe try sharpening them. The renders are rather blurry. Also you need to work on blending.
You also need to work on coloring, lighting and flow, but we can start with just blending for now.
I would like for you to try these tutorials, they will give you a few different styles to start playing with. Most of your signatures look very similar, so let's try to vary it up a bit!
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Tutorial 1]
[
Tutorial 2]
[
Tutorial 3]
Now, there are a few advanced things going on in these tutorials, so don't rush through them! If you get stuck on any one part, let us know and we'll try to help.
I had to wipe my harddrive, so I don't have GIMP atm, but if you get stuck and need help, then i'll redownload it and walk you through it. =)