I think it is hard to change squad art.
I think it works like:
AH checks if one have the required art in squads folder.
If not, it loads it to ones squads folder.
If yes, it uses the existing one (the name of the file is the same for old and rewisited art).
So, one needs to delete the art in squads folder in order to load new one or copy it manually.
Other possibility is that once squad leader have uploaded the art to server it does not rewrite it. But it works for squad leader, since he have changed it manually.
Maybe BS, just my thoughts.