First a warning, when you delete them, they will not show up in the thread they were posted in any more, just the text "Attachment #XXXX". but if you really want to, drag the image to the bottom and click the X in the corner.
With some experimenting I discovered sub folders can be added to the Home folder
Right click anywhere in the "Folders" panel > Add Folder to Home
You can only add folders to "Home" folder
- I created one subfolder named "History"
... and dragged some "old" images from "Home" panel onto the "History" folder
With used images in the Home panel > Click the checkbox in one or more used images
- I think I held Ctrl key (or was it Shift ?) while I selected multiple images in "Home Panel"
- drag the "checked" images onto (say) the "History" folder
When the pointer hits the sweet spot in the Home folder (or one of its sub-folders) the target folder has a very small red-dotted outline. Drop the dragged files onto the sweet-spot folder
Ignore my file names. But I proved to myself it works.
Now I have a tidy "Home" panel to look at