[Dave]

If $show['avatar'] contains something, it will always go into the first if statement first and ignore the rest. So you'll have to unset that variable for it to make it to the elseif.

Edit: it doesn't work because you use the wrong variable in the template.
post.steamavatarfull, post.steamavatar and post.steamavatarmedium is what you should use. The other variable is never assigned to the post array.