Sure thing ;) I won't comment on each signature individually, but I will give you general suggestions for what you need to do to improve.
First off, you need to think more about color. Look at the last one, for example. Theres a desaturated dark green and a vibrant violet (almost hot pink even) right next to each other. Those colors really clash and don't go well together. Take a look at this color wheel:
http://www.sanford-artedventures.com/study...iange_wheel.gif
That's a good place to start when thinking about color. Pick colors that are next to each other (like violet and red) on the color wheel, or colors opposite each other (like blue and orange). Clearly you can't just pick any blue and any orange, then put them together. Hue does matter.
Next I'd say you need to lighten up on the effects. From the looks of it you're taking effects you find cool and just throwing them on there just because. For example, look at the second to last. You have animated glowing text (with some very nasty bevel/emboss type effect), scanlines, various patterns in random areas, and an outer glow around that guy. Use effects when they'll improve the overall quality of your piece, don't use them just to use them. Because of the content (sniper guy looking through scope), at most I would have put some subtle scanlines in key areas. However, I'm not a big fan of doing things like that. Basically, if you can avoid putting something like that in, do so. Only add an effect if it is tasteful and really adds something.
Lastly, don't restrict yourself to signatures. I started digital art a little over 5 years ago and I started off doing "large pieces." I never did signatures. I've probably made as many signatures as you in all that time. Believe me, you'll get much better and you'll appreciate it a lot more later on if you do regular pieces rather than signatures. Obviously this is just a suggestion, but believe me, I've seen a thing or two since I've started :)
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