The first thing you should do is access:

userfiles / plugins / photo and delete all files starting with tmp *. *

There is a known bug (and that nobody pays attention to) in the photo plugin in which any photo upload that is interrupted by the user, or an error, or one of the photos has too many Kb, that is, the wrong format It results in the interruption of the loading process and leaves files that start with tmp, orphaned because no record in the database points to them and therefore they will never be deleted.

There are other plugins like fbconect that generate thousands of image files in its folder, also watch how much size you have wasted there.

You can also write up a quick cron file to remove older images, just let your users know ahead of time in your terms page that images older than xxx days will be removed automatically.  Dont forget to remove the image and the related db records as well.

And inside this cron file you can also add the removal of all tmp*.* files..

+1 patricia, that is great info, thanks :)