They're ok,but dont put big and bulky non matching text that take sup the whole sig,try to center your renders too.
Some fonts i would suggest are
Journal
Birth of a Hero
Linkin Park Text Pack
Also try to use some C4D's (cinema 4 dimension).
If your using stocks (photos with background in them) and using gimp,make a new layer,go to gradient tool > burn and it will turn out with a transition of light to dark.
Overall,i would rate these a 6.5 out of 11.
If your new at Graphics work,good job!
No No No No NO! do not center your focal/render, that looses the flow and the depth. its the rule of 3rd i believe.
here check this site out it will help you with flow/depth/and ligthing the 3 basic parts of making a signature. yours right now you need work on blending effects and all the basics.
http://www.sigtutorials.com/forums/Flow-Depth-And-Lighting-t2539.html











Try doing 500x200.