Altering select elements to include data-img-src
Looking at:

http://api.drupal.org/api/drupal/includes%21form.inc/function/form_select_options/7

It appears there's no way to add your own attribute to the select option from the form builder.

You may need to override theme_select to call a custom select_options handler instead, that checks some of the form data and adds the attributes to the option element.

http://api.drupal.org/api/drupal/includes!form.inc/function/theme_select/7