how to change output html of variable $output on file views-view-field--field-image.tpl.php
You kind of answered yourself already.

open the views-view-field--field-image.tpl.php file, look for that  and add what you need.

however, use the file you can find inside your theme folder ( usually named templates or tpl), not inside the view module.

if you can't find it in there, copy it from the view module. In this case clear the cache for drupal to register the new file.

Always and only edit the .tpl of your themes.