If it's your first image, then I don't think it's wise to start on a wallpaper size.It's not a sig, it's not a wallpaper.. it's just a picture
What do you think? It's my first time making a picture :$
The effect on Casillas isn't exactly eye-pleasing, neither is the "Spains Numero Uno" text. The Casillas text itself is rather creative, bending it to copy the shape of the arm.
I do have to say that you should, in my opinion, stick to making smaller images first, then slowly increase your canvas size into a wallpaper, since wallpapers are a lot harder than smaller images.
Don't let this comment get to you though, as Coach Carter said, this is a good way to "learn from mistakes", if you will. Although I did my signatures/wallpapers on a different graphics site [which is now gone ] detailed comments will always make you a better designer