From Parameters->Configure PokerTH ->Avatar, when you assign an avatar to an AI, I'm unable to remove it.

Thanks for your tremendous work and bye!

It works for me, like this:

Click avatar pic for player 1.
Remove checkmark Select an avatar.
Click OK.

Which procedure are you trying (this one as well?)

What kind of system do you have (pokerth version? MacOS? Windows? Downloaded package from pokerth.net or version from your Linux distribution?)

PokerTH 0.7.1 under GNU/Linux.

It's possible to remove it but there is a something weird. Here is how to reproduce it :

1/ Choose an avatar normally, then exit the chooser clicking Ok
2/ Relaunch the chooser, click on "load an avatar from file", don't choose any file and click cancel
3/ Relaunch the chooser, you're unable to remove the avatar, simply clicking "choose an avatar" and unselecting it. You need to reselect one before, confirm, and relaunch the chooser and this time you can unselect "choose an avatar" which will remove the avatar.

Ah yes, I see what you mean, it's less serious than being actually unable to remove an avatar, but yes it's a bit clumsy.
Now I have to try to concisely describe this so I have a title for the bug report