If you copied the file(s) where the linked topic indicited, it might be that you haven't really restarted the game. You need to exit completely (by bringing up the recent apps and swiping The Quest off screen).
Please also make sure the bmp file(s) are in the proper subfolder, so "Android/data/hu.redshift.thequest/files/data/worlds/bres" and named properly ("head01.bmp", "head02.bmp", etc.).
(Also on more recent Android versions it seems you don't have access to this folder anymore ("for your safety"
), so it might be that you can't copy anything there. It might look like you've copied the files but they won't get into the proper folder. Custom portraits is not an advertised functionality, just something "extra". If it's working, nice. If not, well... Sorry.)