In fact you could just go into standard_text_editor.tpl and edit the line below "{if $allow_images}"
instead of
onclick="showHTML('image','image-drop')";
just put
onclick="DoPrompt('bbcode','Enter full image URL.','[imgpop]','[/imgpop]');"
Do mind I suspect an error in the $allow_images code somewhere as the button is ALWAYS show regardless of permissions. Also any checks on total number of images will be GONE as you're no longer using the IMG tag this is checked against. Also users that aren't allowed to insert images will still be able to do it through the imgpop tag. But it's a quick and easy edit to make like this. 
