Hi Pierre...
Yes, but how to change?
I dunno. I think my problem is that there are two trees and two sets of checkboxes whose functions overlap, but only a little. One tree's purpose is primarily navigation and filtering, the other's is assignment.
Suggestion
Since a tree is
very meaningful for navigation, but
less meaningful for assignment, my suggestion would be to
change how the categories are displayed in the Properties/Category tab.
Instead of the tree, a "flat" list of assigned categories could be displayed there with an "Add" button.
The "Add" button would "pop up" a category
management and assignment dialog which would display the category tree (with the check boxes for assigning categories to images) and also show buttons for category add/delete.
One objection to this idea is that it slows the assignment process down by requiring an extra click or two.