Take Two. Not really happy with our current baking API tbh, there are many things that look messy
(even inside Blender code, how can a func named 'ED_object_get_active_image' can be reserved to new shading system???),
but this should do it for now. Just avoid too much exotic node systems with Cycles and this addon,
and/or clearly select/activate the image tex node containing the picture you want to use as target.

Based on a patch by Edward Gabriel Rowlett-Barbu (Zadirion), many thanks.