A quick sloppy non pro way would be to just open it up in photoshop, use the magic wand on the white border, and hit delete. Umm Make sure the tolerance isn't so high that it catches some white stuff on the character too.
If the character has too many white stuffs on them that gets caught by the wand. Select the outline of hte character using the magnetic lasso (you can use other stuff too but the magic lasso is pretty noob friendly imo). It snaps to colour changes, and "saves" your progress as you select an area, if one of the checkpoint thingies is incorrect, just push delete and it'll back up a checkpoint, easy. But yeah, then just select the portrait using it. Then in the select menu click inverse (I think Alt-I works too, don't remember, been years lol), then delete.
If you don't have photoshop, beats me.