Copy/pasted from the 4chan Dungeon Crawler General since you guys deserve this knowledge I dug up.Steamfag here. As you know, Ghostlight was retarded and in their release of Elminage Gothic, they removed the ability to have custom portraits.
←— NEENER NEENER NEENER!
Seriously now, here's a guide to how to do it:
http://steamcommunity.com/app/291960/discussions/0/619568192935281666/It involves actually overriding the files but it isn't complicated. I'm going to toy with the files tomorrow and see if I can add more to the list so that this doesn't happen but it seems unlikely. A lot of the data files have entire columns and rows labeled as ??????? which is completely useless compared to some Japanese.
You can also have custom music by simply renaming a file to whatever file it needs to replace. For example, the title screen theme is EL3BGM_01.ogg
MP3 wont work, it must be .ogg