I think that onion skinning is low down on the list of priorities for the image editor. The image editor/animation editor is the weakest part of C2 now and is in need of some love <img src="smileys/smiley27.gif" border="0" align="middle" />
It needs to be streamlined and a more elegant gui created for it. Curently there can be up to 5 floating windows depending on what is selected. This is incongruent with the rest of the C2 interface which is a non-blocking UI.
If the image editor/animation editor had a mini timeline with play/scrubbing/transport controls with integrated animation list and preview it would be much nicer to work with. My biggest beef with it currently is not being able to marquee select several animation frames to delete or shift around which can be a big pain.
Also replacing animations with a new animation is very clunky. You either have to replace in folder (which I prefer to not do), or delete all frames and re-import, or create a new animation with a dummy name then delete the old one and rename the dummy name with original name. Would be much better to have a replace animation option like you would in After Effects.
I also do my sprites in external programs, but having a more functional sprite editor would make prototyping a more pleasurable experience :)