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When you double-click an image in CKEditor fuel pops up a dialog where you can edit its attributes.
It seems "class" is ignored.
After line 51 in https://github.com/daylightstudio/FUEL-CMS/blob/master/fuel/modules/fuel/assets/js/editors/ckeditor/plugins/fuelimage/plugin.js we need to add:
className = element.getAttribute('class');