ok short version heres what you need.
put the picture you want in paint/photoshop
remove the BG that you don't want showing
then using irfanviewmake sure every color that you do want showing has 10+ in RGB (at R:10 G:10 B:10 it'll still look black but it won't register as such in FF)
save as .pcx
then open the character you want to change the portrait of and click switch image and replace it with you new portrait.