I mainly delete images (after FB is received - or 90 days has passed) just so I don't have to scroll through them all in my Auctiva.
I've never checked the closed auction to see if the pics are gone from that, because I really don't care about that.
I go to manage images, find the folder the images are in, delete the individual images and then go back and delete the folder.
I always have a copy of pics on my 'puter, anyway and I delete them when I'm positive I won't be needing them anymore.
So, as Mike D says - it just makes my own image archive easier to navigate, and that makes me happy.