Digital Photography ~ Critique plz

[drink more ovaltine]

Started this relatively recently, never taken a class before, wanna give me some tips?
I shoot with a D40 and I use natural lighting. All editing is done on GIMP.  

[Neon_Dinosaurs x3]

Theyre very good for not have taken a class. But, the first too vignetted. For the second, try taking out the rock on the left, it distracts your eye and doesnt let your eye flow through the photo as easily, and maybe crop it slightly on the left.The third is really nice, I like it. Try bringing out the texture in the wall though. For the second to last, the spot just above his right wrist is really bright, trying making it less bright. And the last, I like ALOT. Its slightly centered but, because the model has things to balance it out it works.(:  

[Agent Mallard]

There's a lot of negative space in your pictures. I found myself looking at the background more then actual subject of the picture.