Please note that these instructions were pulled from Nintendo-Play's Super Smash Bros Stage Thread constructed by pixilovesbunnies. Whatever I have edited myself is in green text. Also, any downloaded stages sent to your SD card can be edited in-game. Hope this helped.
Also, try to upload pics of your stage along with the stage itself. Here's a decrypter to make SSBB pics on your SD card visible on your computer:
http://brawl.kakkoister.com/