to let you guys know my current thinking about movable objects.
i was thinking about lots of ways to do it, then i discovered that it is possible to embed arbitrary data in a .tiff file along with the image.
photoshop for example can embed full layer data in .tiff files.
i think the best way to do this is, when you add objects, it will automatically (unless you tell it differently), save your files (rename) as .tiff files. then it will be able to retrieve objects so you can always edit them, move them, etc., but still view the images in any image editor.
you can then do a save as.. to save it in another format (and lose ability to move objects).