It took me two days to figure this out, so I am going to post this in
the hopes of saving someone else some time troubleshooting this problem.

A few weeks ago, for the first time I ran into the problem where the
server would delete the image at some point while it was being uploaded
to the server. I eventually discovered that the volume was out of
space. Silly me :)

A few weeks go by, no more problems. Then, out of the blue, someone
from another department has this problem. First thing I do is look at
the free space on the volume. It has something like 40 gigs left. The
image is around 2.5 gigs, so that wasn't it. This went on for two days,
and I had run out of ideas.

Turns out that this department was "borrowing" space on my imaging
server. When I agreed to temporarily give them space months ago, I set
a quota on their images directory so that it would not exceed 25 gigs
(they have a tendency to fill up our servers with a lot of uses
garbage). Guess what? That last image they kept trying to back up went
over that limit, so about 1.7 gigs into the backup the service would
delete the image file. I had them remove some of the images from their
directory to free up some space, and the problem went away.

I would be surprised if this is the cause for anyone else's "auto
deleting of images from the server" issue, but it wouldn't hurt to check
and cross it off your list of possible causes.