You are right for the Tree.many2one_image widget !
I must admit that I never used field.related before ;)
It acts like the one I created.
But for the Form.many2one_image the purpose isn't the same.
The Form.many2one_image represent a M2O object, so you can change the M2O relation directly, with your solution you will directly change the showed image itself and not the M2O relation.
You are right for the Tree.many2one_image widget !
I must admit that I never used field.related before ;)
It acts like the one I created.
But for the Form.many2one_image the purpose isn't the same.
The Form.many2one_image represent a M2O object, so you can change the M2O relation directly, with your solution you will directly change the showed image itself and not the M2O relation.