Ah, that directory is for
quest opponent icons.
The player and normal duel avatars are defined in the skin files. The ones you normally see would be under the
default skin although different skins can define new ones. In other words, you need to navigate to
res/skins/default, find the avatars image file (IIRC, it's called "
sprite_avatars.png", can't check right now on this computer), and edit it. It's basically a bitmap of 100x100 tiles, and you need to place your new images in that grid. You can replace images you don't want or increase the file size and add new ones. Just keep in mind that the grid size must be increased in steps of 100 pixels.
This probably isn't really documented anywhere but maybe it should be.
