Views load the whole entity even though they only need 3 fields
Are these custom entities or from a module? If they are custom you could try to override the 'query' or 'load' function in the entity controller class (defined in hook_entity_info() ). If it is from a contributed module, there isn't anything you can do without altering the module code.

Also please note that the actual images are only loaded when the browser requests them over http via an  tag, the entity loads the fields over SQL. Agreed, this creates some overhead. But I guess you would hardly notice the difference when a few fields less are loaded.