[cellarius]

Correct, no more template edits. Please check the following:

• make sure you have uploaded the icon file images/buttons/cel/cel_photo_popup.png
• then check whether it works in unmodified default style
• if you are using a custom style, it may be that this style uses its own images directory; if this is the case, make sure to upload cel/cel_photo_popup.png into the buttons directory of your custom style. Example: images/mystyle/buttons/cel/cel_photo_popup.png
• If you don't know where to look for this directory, check stylevar imgdir_button to find out.

So, why do I use the buttons and not the editor directory? Because, funny enough, the stylevar for images/editor is not available in the editor plugin javascript environment, while the one for images/buttons is.