If, for whatever reason, an image is corrupted it just won't load; it won't stop the forum from loading at all (forum data is all in the database).
I know of several clients who have massive (5gb+) upload folders.
The only mechanism we have for separate folders is the upload folders for avatars and one for files; there is no mechanism in place to upload to different folders.
You should be able to upload until you run out of disk space or are at the maximum number of inodes (on some hosted systems).
