Hi Alex,
We have a site with this same challenge, its a lost and found pet database and each pet’s listing can have an image. We did two things to help cut down on the number of images collected.
http://www.dovelewis.org/lost-and-found-pets/
One is that all listings auto-expire off the site after 6 months. We have a date conditional in a weblog/channel tag that prevents entries older than 6 months from displaying or being included in searching of the database.
The second is that we add an image upload date to all image file names. This way, we can go into that folder once or twice a year and delete all images with upload dates older than 6 months, and that helps keep our images folder a little leaner.
It would be great if we had the option of having some files (like these images) be removed from the server when an entry is removed and would make our life easier when it comes to keeping our image folder on the server a little smaller. Now that all images are assigned an ID number with the new file manager in EE, it should be possible for to do this.
I would recommend you create a feature request and site this thread in the request.