empty ng-src doesn’t update image

The answer to this is in the Angular code. It’s not a bug, it’s just the behavior they decided they wanted. Starting on line 13895, you can see this directive code:

forEach(['src', 'srcset', 'href'], function(attrName) {
   var normalized = directiveNormalize('ng-' + attrName);
   ngAttributeAliasDirectives[normalized] = function() {
   return {
      priority: 99, // it needs to run after the attributes are interpolated
      link: function(scope, element, attr) {
          attr.$observe(normalized, function(value) {

         if (!value)
             return;

         attr.$set(attrName, value);

         if (msie) element.prop(attrName, attr[attrName]);
       });
     }
    };
  };
});

The key is in:

if (!value) return;

So as you can see, if you change the ng-src expression to an empty string, this will never change the actual src value. You can get around this doing something MadScone suggested.