Custom geometric information related to an Image could be managed in the NamedValueSet returned by Image::getProperties().
The “only” missing building block would be to have those properties serialized and loaded back from the PNG file, perhaps as a custom “chunk” in the metadata. That would mean that they “travel” with the image file and could be authored by the GFX designer.
This other thread seems to suggest a small step in that direction: Get access to libjpeg & libpng
It would be nice if @jules or someone else in the ROLI team could consider this for implementation