Yes, get Brawl Box and common5_en (both are in the resources section of the vault) use Brawl Box to open the common5_en, inflate (I think) char_bust-tex, then lz77, then browse around till you find the character's preview pic that you want to change. Right click and extract it, then open that picture in something like Gimp, or Photoshop and replace it with a picture of the texture (from a screenshot or something. Index the image and save it, then back in Brawl Box, right click the portrait you want to replace and click replace, then select the one you made. When you're done, save the file as just common5.pac (no _en) and put it in your SD card in RSBE/pf/system.
I've understood everything but indexing images. How I do that?