Avatar Portraits

Perhaps this is a silly question, but I haven't seen it answered anywhere. Am I able to change the avatar choices for the game? If so, is there a folder somewhere, a file proportion and type to use, etc?
High Quality Resources for Collectible Card Games
https://www.slightlymagic.net/forum/
https://www.slightlymagic.net/forum/viewtopic.php?f=70&t=30192