I've not really tried to use the "Thumbnail (optional, overrides automatic thumbnail generation):" option before, when uploading an image - but I've just been trying it, and can't seem to get it to work.
I'm using 2.2 and all my images are stored in file space not the database and I'm simply specifying a different and smaller .jpg file in the thumbnail override field when uploading an individual image. Whenever I try it the alternative thumbnail however is ignored and the resultant thumbnail is generated from the original.
Has anyone else tried this recently? Is there perhaps a special format for the thumbnail or is using file space rather than the database making a difference? Or am I just being stupid