Take a picture from your character (Via Brawl, etc), and save it on your SD (It'll be saved as a .Bin file in Private\Wii\APP\RSBE\al\), use this:
http://filetrip.net/f1752-Wii-SSBB-Image-Converter-Bin2Jpg.html to convert it from .bin to .jpg
Then edit it on Photoshop (Eliminating the font, etc(Using a layer so it'd be invisible, and Index Color 256 I think) and save it as .PNG when done (Size 128x160)