I can't see the "Change Avatar" menu link on my...

Your members should change profile avatars on the Gravatar.com. The “Change Avatar” menu link is hidden because BuddyPress system doesn’t allow to have square avatars and horizontal group thumbnails. There is only one parameter which defines height & width of avatars and group thumbnails.

Anyway if you really want to let your users to change their avatars on your website then there is a small trick.

First you should create all your groups, then open functions.php and replace:

define( 'BP_AVATAR_FULL_HEIGHT', 135 );

for:

define( 'BP_AVATAR_FULL_HEIGHT', 215 );

Now paste following code in the style.css

#change-avatar-personal-li { display:block!important; }
.group-box img, .single-group-box-image-container img { height:135px!important; }