Make the image resource URL editable, so you aren't restricted to select only webresource images.
Now you can only select webresource images for the buttons. Make this field an editable text field, so I can reuse CRM images, which are located below the '/_imgs/...' folder.
This has now been implemented as of version 126.96.36.1999.
Thanks for your continued support and feedback!
Happy Ribbon Editing!