Plug your phone into your computer with the usb cable. It should pop a menu with 4 options, select mount as disk drive and hit done. Go to "my computer" and look for the drive called "removable disk" this will be your SD card on your phone. Double click on it, then double click on "DCIM". It will open another folder and inside that open "100MEDIA". Take the picture you're wanting to use and drop it in that folder. Disconnect your phone from the usb cable and wait for the SD card to mount. Then go into your gallery and find the picture (it will likely be under camera shots and/or all pictures). Long press on the picture and it will pop a menu, select "set as" and then "wallpaper". You can expand the green box it shows by dragging your finger over one of the sides, and position it where you want it by placing your finger in the middle and dragging it around. Hit save, and it should be done!