Are avatars saved in the same folder as uploaded images?
Gravatar ones should still working, isn't it?, so new users should use it instead.
Discourse uses some kinda database blob.
Yes. That would be correct.
Databases for files ?
Well.. a workaround could be to revoke the writing privilege to the table used to store the images if it's different than the one used to store the avatars.
I can't change my gravatar, though.