Hi, Mike.
I got a response to my techincal help request. Apparently there is a known issue with this which has been reported by several users.
I imagine there are a lot more users who haven't reported this as deleting old pictures is not a priority for many users!
However I think that this is potentially quite a serious problem as users experiencing any difficulty with this will simply not bother to delete any unused images. If my case is typical this means that there are hundreds of megabytes of wasted server-space for each user, and it will get worse as time goes on. This will eventually become a major problem.
I speculate that the amount of undeleted images already accumulating might have some connection with the loss of image hosting services last weekend.
Even once the problem is addressed, there is still an issue with 'lazy' users. With no reason to spend time deleting old images there will be many who simply will not bother.
I think it will become necesary to introduce a purging regieme by where image files not used in listings will be automatically deleted after a period without user intervention. For example you could list images not used for six-months at login, and ask the user which (if any) are to be retained. Those which are not retained will be deleted one week later.