Displaying avatar as default image

There are literally 100's of places that the cover image can be displayed. You are going to have to be specific about the exact location (Page, area on the page etc). Screen shots come in handy. Also, if its one of the LAYOUT TYPE (Article View, List View, News View etc), that also is information that I'd need.

Each LAYOUT has its own specific set of rules and options. List View layout for example will first check to see if there is a Cover Image and if there is, it will display that. If there is no cover image, it then checks to see if there is a Category Image and if there is, it will display that if there is no category image, then the Users Avatar will be used. Other layouts like News View II, Tile View etc first check for a Cover image, then they check for a Category Image and if neither of those exist, it uses a No Image place holder that is sized the same as the large thumbnails. Article View and News View check for Cover Image and Category Image and if neither exist, they don't display anything. Those layouts are specifically designed that way.

The only layout that is specifically designed to handle the User Avatar is the List View (as list view uses the avatar helper to display the cover image or category image). The other layouts are based off the Thumbnail Dimensions for AMS (which don't match the small square avatar), so you'd end up with out of proportion blurred avatar images.

As far as how to do it... it requires customization. You'd have to edit the specific template(s) and modify the template code that outputs the cover image. You can see how this works view the List View layout template (the template conditionals).