Hi Rune!

Try this:

File: /Umbraco/Js/umbraco.services.js

Code Line: 6306 Note: This is Umbraco 7.4.1 so the line might be different, but just search for the funtion in that js file.

Function: insertMediaInEditor

               var data = {
                   alt: img.altText || "",
                   src: (img.url) ? img.url : "nothing.jpg",
                   rel: img.id,
                   'data-id': img.id,
                   id: '__mcenew',
                   class: 'slimmage'
               };