It would be nice for plug-ins if the set of unique identifiers used for "images" did not intersect with those employed for "items", thus permitting a 'gimp-image-is-valid' test to provide a correct response if passed the identifier of an item.

The reasoning is that there is much similarity between the structure of a "layer group" and the structure of an "image"; at least with regard to the constituent of "items". Or put another way, an image's drawable hierarchy is identical to that of a layer group (please correct me if I'm mistaken) and it may prove useful in the future to make use of this similarity (e.g., load image as group, save group as image).

Currently, images and items can share the same unique identifier and my proposal is that the identifiers for both be generated from the same pool of available identifiers, guaranteeing that no overlap takes place. This fix would seem pretty trivial to make -- and I can't think of any downside (other than perhaps exhausting available unique identifiers in 8000 years instead of 9000) -- however, I was unable to find in GIMP's code where these identifiers are retrieved.