well, you may be a paint.net noob but you are showing that you have experience with some graphic design. I am wondering how much of this you did vs how much you pulled from here and there. Don't get me wrong, I am a HUGE fan of pulling from multiple sources and mix/matching.
Just hard to critique is all I am saying, not knowing what the origonals looked like.
Only thing I woudl say is get some pluggins if you haven't yet. Check out the pluggin page here. Get all of the pyrochild and EdHarvey ones to start with