Bootstrap popover, image as content

As it currently stands your this is referencing the current scope, whereas you want to be referencing the element which the Popover instance is attached to. This is done simply by wrapping the expression you have for content in a function:

$('a[rel=popover]').popover({
  html: true,
  trigger: 'hover',
  content: function () {
    return '<img src="'+$(this).data('img') + '" />';
  }
});

See demo:

Plunker