25GB is just barely enough. If you have many images you’re going to have trouble. The easy thing is to get a bigger droplet. The next-easiest thing is to add a data “volume” and put the images there. Just how to do that requires some knowledge about unix filesystems and docker mounts.
There are several ways–put the extra space at /mnt/whatever-they-giveyou and then copy the images from /var/discourse/shared/standalone/uploads to the extra space and then adjust the app.yml to use that path for /shared/uploads.
The safe way to do it would be to spin up a new droplet so if something goes wrong you haven’t trashed your site.