When trying to upload a background image, I get the following error:
Error in query [INSERT INTO images (imageid, name, imagetype, image) VALUES (42, 'Hintergrund Netzwerkplan', 2, '...')] [Duplicate entry '42' for key 'PRIMARY']

Looking into table "images" shows me that the last image with imagetype "2" (background) has imageid 41.
But there are icon images (imagetype "1") up until imageid 228.

So it seems to me the import logic does something like "select imageid from images where imagetype = 2" instead of "select imageid from images".