Well, Simple I make it by using only from brushes and stock photos.
Before doing anything,
Pick a character to whom you want to represent the character within..
That time, I was thinking of my close friend AdoreDog,so I catch up the idea with Amu.. :D
First, Search for the suitable background,so that It can blend the character of the wallpaper inside.
This can be found at www.deviantart.com Or maybe you can search it in google with keywords fantasy or else (that may bring a vivid and unusual effect for your wallpaper)
Second, Choose a theme of your wallpaper..
It can be goth,fantasy or else.. :D
From this theme, you'll get a clue for the colour theme
You can set the background you had chosen by pressing Ctrl + U to change the entire colour harmonies..
Third, Decorating time!
It's the time to place the brush into the wallpaper. The brush can be found on google as usual.. :]
From my own experience, using the brush well needs some time and practice,
If you wonder how the wallpaper can be such like that,maybe the answer is the influence of practicing.
The brush can be whatever you like according to the wallpaper theme.. :)
But,I have to remind you this important thing...
~Wherever you pick up the brushes,please DON'T FORGET to put on the credits to the creator,so you aren't equalized with plagirizer..~
For some people, brush is kinda a make up for the whole face,that perfecting the wallpaper more.
This means that the other wallpaper with no brushes looked uncomparable as the one with brushes! But, some people have considered the same thing, that a true / better wallpaper is the natural one, that doesn't have many brush influence. If you do, you have already a master in photoshop..!
Honestly, I'm still trying to perfecting the style with no brushes at all,so I am not a Master yet..
Well, You and I, both need more practice again I guess..
I hope this really help.. :D