What computer do you have? If I find the time I'll do it for you, but here's how I did it anyway:
1. Find the icon of my computer in Macintosh HD/System/Library/CoreServices/Coretypes.bundle/Contents/Resources
2. Duplicated it, made 2000 backups, etc etc.
3. Used iConveter (Google it, it's online) to convert the icon to a 512x512 image.
3. Opened Photoshop, cut out the screen, put the wallpaper in it and then traced round the 'sheen' with the pen tool. Then I made it white and played with the opacity for a while until it looked right.
4. Saved as a .png, then went back into iConverter and converted the png to an icon again.
5. Gave it the proper name, dropped it back into the folder in step 1, then restarted.
6. Ta-dah!
I'm only 13 and can do it, so anyone can