I believe this to be a Windows TS3 Client Bug.
I experienced that problem too after setting an avatar on my testserver and then trying to change it again.
It seems like the client cannot delete the cached avatar file in the following folder :
Code:
[HDD]:\Users\[USERNAME]\AppData\Local\Temp
[edit] : maybe I should add that this is a hidden system-folder, however just typing the correct address into your address bar should take you there
Manually deleting the file named :
will enable you to change your avatar again.
Probably only once tough, because the cache file will be recreated, so you will have to delete it every time you want to change your avatar.
(its just a simple image file without any extension, adding the extension will reveal that its your avatar)
I hope that this can be fixed in a future release.
(I didnt see any bugreport forum/thread so i'll assume that the devs will read this too

)