I like them, but how long have you been using Paint.Net?
My favorite is probably the first, it looks almost like "burning metal."
The second one I noticed in some areas there are little white pixels never mind, that might just be my browser, because when I copied it into PDN to circle them, they were gone. (Does anyone else see those white pixels), and it's a polar inversion based image, which I somewhat dislike, because the effect makes everything look better, so theres little effort.
Third is very nice, nothing to say about that.
Fourth I like the least, because theres no depth (Do you know what I mean? I'll explain if you don't) it it.
Good work!