Not bad, but it looks like mostly just brushing. I think that what you need to work on the most is you blending and text. Also try and work on smudging and filter effects to give the background some more depth.
I like the last one with the tank, the render and background really fit together, nice work on that one
Yeah, brush at this point in time is mostly all I can do. I'm not so good at the whole depth thing yet.
Don't worry you'll get the hang of it, just start experimenting with different effects and what not and you'll have the hang of it in no time.
It's all good, man. Your style reminds me of old school grunge. Do some tutorials, download some of the resources available here in the resources thread, and get crackin'. You'll improve! I also like that tank tag. Good compo and coloring. ^_^
Yeah, C&C3 owns. <3 (The tank is the GDI mammoth tank from C&C 3:Tiberium Wars incase anyone didn't know/was interested in the origin)
Asin work on everything, heh. Anyways, just made a new one...it doesn't really blend well...but I like shiny stuff.
Blending and text. Like some of the others said, follow some tutorials, but don't be afraid to experiment too, that's how you get better, you find your own style. Just keep at it! This site is a great resource for numerous tools to help you get better, so make sure you use it to your advantage!