iphone-hacks.com

•For 1.1.3
For Summerboard:  (*It has to be named "Wallpaper.png" in order for it to work)

/var/mobile/Library/SummerBoard/Themes/*Create a Folder*/Wallpaper.png
Alternate:
/private//var/mobile/Library/SummerBoard/Themes/*Create a Folder*/Wallpaper.png

For the LockScreen:  (*It has to be named "LockBackground.jpg in order for it to work")

/var/mobile/Library/LockBackground.jpg
Alternate:
/private//var/mobile/Library/LockBackground.jpg

•For 1.1.2 and down

For Summerboard:  (*It has to be named "Wallpaper.png" in order for it to work)

/var/root/Library/SummerBoard/Themes/*Create a Folder*/Wallpaper.png
Alternate:
/private//var/root/Library/SummerBoard/Themes/*Create a Folder*/Wallpaper.png

For the LockScreen:  (*It has to be named "LockBackground.jpg in order for it to work")

/var/root/Library/LockBackground.jpg
Alternate:
/private//var/root/Library/LockBackground.jpg


*You will need AFPd or SSH in order to install them.