...Can be pretty difficult teaching someone 'design sense', ya either have it or you don't. I think you do have it in this case, so there's really nothing for me to offer that would drastically improve your work. What it does lack is a refined look. By refined, I mean take a look at some of the better graphics artists out there [GT3?] or Danillo's work. Take a peek at Danillo's wallpapers. What, besides clean graphics do you notice about his work?.. His wallpaper work [to me at least] looks to be one design, one cohesive mass of graphics and not just a bunch of graphics that comprise a wallpaper. Does that make sense? I'm currently running into issues like this making my first WindowBlind. I need it [ok, want it] to look like
one windowblind, not 1000 graphics thrown together. Make sure your wallpapers maintain at least some semblence of continuity.
I've had people ask me if I draw some of my designs ahead of time, or have a "vision" of what the final product will be beforehand and to be honest, 99% of the time it's entirely improvised. Hopefully the result LOOKS cohesive enough to fool the user into thinking I intended the wall to look as such from the start. [but I know better

..]
So all I can suggest is keep playing. Patience, practice and tenacity in photoshop
will yield results. [or whatever program you use] You'll get it in no time.