Avatars can be an image of whatever suits you. It is advisable to have one so that you are more easily recognized but putting one is completely up to you.
Avatar pictures have to appropriate for all viewing audiences. Meaning no questionable content. Feel free to use the avatars in the gallery provided by the forum host, however not all avatars in the gallery are 'advisable' for this forum, we (k9academy staff) don't have control over what is showed in the gallery, only the forum host does. So please have common sense if and when you decide to use one from the gallery.
All pictures are automatically resized to 90X90, pictures cannot exceed a certain file size or else they cannot be loaded. Should this happen simply downsize your picture a bit and try to load it up again.
Have fun with this!
Steven