When creating your own avatars, create the avatar and resize it to 64X64 pixels and name it according to the convention you see in the avatars. Then resize the avatar to 32X32 and name it accordingly but adding the i to the name. Upload both images to the avatar data folder.
Unless you wanted to go through and change the template handling the avatars, it is best to just use the Boonex sizes. The divisions holding the avatar will also be set to either of the sizes mentioned.
Now the one change I did make was to switch to the 64X64 size on the member submenu; one of the strange decisions by Boonex was to use the 32 image.