The first one (a banner I think) is really good. The text fits it well and the halo 3 logo creates an effect that seems to 'light up' the spartan's visor.
On the 2nd one, I like how some of the insignificant text fades behind the alien. The only part that seems to be a little unfitting is that the alien doesn't blend in too well, but that could be seen as a good OR a bad thing. Other than that, the color blending is done really well.
Keep up the good work man.
PS: Gimp is teh ub3r pwn.