I want to be able to use a different image for the thumbnail and full image. The primary purpose of this would be to show a crop of a face in an infobox but show a half-length portrait when it's clicked. Another interesting use can be seen here: https://en.wikipedia.org/wiki/Napoleon#Metric_system. When the gold coin is clicked, it shows both sides.

On English Wikipedia this is achieved using CSS image crop. This does mean the full view is downloaded for the thumbnail (like https://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/France_1803-04-A_20_Francs.jpg/410px-France_1803-04-A_20_Francs.jpg) but never fully shown. It's also not possible on all projects.

Something like [[File:Gold coin both sides.jpg|File:Gold coin heads.jpg|thumb|Gold coin, click to see both sides]] would be nice.

Alternatively the thumbnail could perhaps show the area of an ImageNote as defined on Commons. So it would for example be like:

[[File:Gold coin both sides.jpg|area=heads|thumb|Gold coin, click to see both sides]]