Thank you @AFRLme and @redspark for the mention of the image encoding/game optimization
Wiki entries; I was aware of these, but of course a second read, does more good than bad.
@redspark, the thread was spot on! You put my searching skills to shame, - I thought that I had searched the forum thoroughly before creating a new thread! My bad :/
@Machtnix: Thank you for the lead! After reading your answer and the quoted thread (above), I'm still a bit confused though...
In the thread @Marvel says that upon export, the images are automatically trimmed. This should had been enough to assume that we can (and for practical reasons, should) export scene objects and their animations (even if they are 20x20px small), untrimmed: At the full scene-size, with lots of transparent pixels (graphics fat) surrounding the objects. In other words, PS/Gimp "layers to files".
However you advise against doing that - am I right? Or are you referring particularly to scenarios such as the example of the "river+leaves+ants" background-animations, where the content might be scattered around the scene and thus can't be trimmed sufficiently when the game is being exported?
Thank you all for the help!