The MarkItUp editor JS in the WYSIWYG module doesn't allow for icons provided by a custom module.
Steps to reproduce
Attempt to use the WYSIWYG MarkItUp BBCode module I've put into a sandbox. The YouTube button will not render, even though it's properly providing the icon path and file per the WYSIWYG API.
Alter the markitup.js file that is included with WYSIWYG module to respect icon paths and files. Patch incoming.