System memory usage after closing all imagess

[Ofnuts]

So, your fist image shows that there are no "user" images (since you have WIlber's head) but there is a pretty big image remaining, so some script/plugin must have created it, possibly by duplicating one of your "user" images. The undo history is empty likely because the undo stack has been disabled on that image (routinely done on such images since the users won't be able to Ctr-Z on them anyway, and this spares memory and CPU time).