This means that images' ownership is not given to caller in particular.
libgimp will now keep a reference of all GimpImage-s it creates and
return this same reference if called again. It also means that you can
now compare images by pointer comparison (as 2 GimpImage objects
representing the same image ID will be equal).
Obviously as a side effect, gimp_image_list() is changed to (transfer
container) as you must only free the container now, not the elements.
Also various other functions creating new images are now (transfer none)
too.

Long-time plug-ins will have to be taken in consideration in a further
step (we currently never free GimpImage for destroyed images in
particular).