Show is better than tell:
Often, when viewing images, firefox "caches" the image in order to be able to load it faster when visiting that site again. Left unchecked, this cache (of images and other assets) can pretty much infinitely grow. Many other apps also have big caches.
Bleachbit actually is useful. Instead of hunting through your system and accidentally rm -rfing the wrong folder and losing all your precious firefox profile data, it enables you to quickly nuke all caches, freeing up a significant amount of space. I would probably free up 15gb+ if I ran it based on these images.
EDIT: just ran it. I freed up 6gb of space. Not 15gb. Huh. Still, pretty good though, and if you are space starved (I used to use a machine with only 32 gb of storage TOTAL), then it's useful to keep things slim.