Replace RefineryCMS behaviour to add external images in the WYSIWYG editor. Remember to add it in 'initializers/refinery/core.rb' through the 'config/register_javascript' option

refinery_external_image.js.coffee

$(document).ready ->
   $('.wym_tools_image a').click (e)->
      e.stopImmediatePropagation()
      e.preventDefault()
      id = $('.wym_iframe').find('iframe:visible').attr('id').slice(-1)
      url = prompt('Insert external image link')
      img = new Image()
      img.src = url
      alt = prompt('Insert alternative text for the image')
      wym = jQuery.wymeditors(id)
      wym.insert('<img src="'+img.src+'" alt="'+alt+'" width="" height="" />')