Custom CSS style to upload in WYSIWYG field

Hi guys,

I've tried adding some custom CSS styles using the wysiwyg_custom.css and .php approach however I can't seem to get it working. Ideally I'm trying to add a CSS style so that when an image is uplaoded in a WYSIWYG field, they can apply a class as they won't be able to manually add it using the source code button.

Any suggestions greatly received.