I am just posting this to just see what people think. I really suck at GIMP, I don't know how to use renders or anything, so this is what I came up with:
i find that the backgrounds dont really match the renders at all you should see what u can do with changing them, and in the first one the r4edner of the spartan is a little blurry.
The first one looks really unnatural because you can see where you cut and pasted. Blech. In the second one, the sword is kind of coming out of nowhere and you can probably fix that as well. And in the third, again with the random floating weapons. Also, you should add some C4D's. They look a little too plain.
Not bad for your firsts but they could be much better. For one, the sword should be an accent to the second one, not the main render. So for that one I would suggest rotating the sword so it is vertical and lowering the opacity until it blends w/ the BG. Also make MC bigger cuz he's not very noticable and isn't really in the sig too much. For the First one it just looks like a poor version of the stat sigs. So to make it good take out the BR or find a better render of it and make ur guy bigger cuz he's not the focal. Also the text doesn't fit. On the third one if you made the BG good job, if you didn't then it's just 3 renders layered but w/e. The third is the best but the BG needs a light MC Green to MC Visor Orange Gradient map with a low opacity to make it blend a little better. The SMG doesn't work though. So since ur not experienced they're okay but there is a LOT of room for improvement. If you like making sigs I would suggest staying on this topic and just post ur art and get CnC (Comments and Criticism) to make you better. So thats all I've got. Good Day Sir! Edit: I hope it doesn't sound like I'm flaming... I'm not trying to. :/