First of all, so far your designs have the right idea, just the wrong execution. I attribute this to over 'posterizing'...
This to this is not a good look, imo.
'Posterizing' properly depends on the style of your piece, I don't do it much at all. So I can't show you a good example of my own. First of all I'd like to know what your using for graphics? I'm using GIMP at the moment and it's very limited compared to PS but you can still get some decent sigs out of it.
This is your best piece so far and it has potential. All it really needs is to look a bit cleaner, the one things I like to have in my sigs is the look of high quality. I try to work with pictures with high resolutions as much as I can, sometimes high res pictures of what you want aren't available. One thing I try to do is sharpen pictures and play with the levels/brightness contrast to give it a higher quality look. After you've improved your visuals you can work on text. My early work was mostly white/simple and uninspired looking text and although I got great feedback on those sigs, you should never settle and always try to keep improving.
These are two sigs I made in the first month of Graphic making. Visuals were good but the text was always boring and white. Once you start making dynamic text it'll be fun trying new things and playing around, it was for me at least. :
I will use your SHIELD sig to give you a quick example of how I would make it look:
It's simple but it's a smoother and higher quality look, which is more my style. It's not for everyone but this is just an example of my style applied to your signature. Not fond of the font as I would go with something more modern or with a military style. That text is quite simple but with a few blending tricks, it'll give a bit of life. The 'posterized' look you went for could be executed in a way that looks really fitting to The Shield. A gritty feel, with more textures applied on to it. I could give an example but I'm short on time right now.
Once you let me know what program you're using I can give you more detailed tips via PM, mostly if you're using LOLGIMP like me. This is probably common knowledge by now and this is my personal preference but I don't watch tutorials, I like to experiment, it's not for everyone. Other artists could probably direct you to a few Tut's they know about. I usually don't give advice as I still consider myself a bit of a noob, but other more experienced guys can weigh in as well.
That's it for now.