A thumbnail is an incredibly rough initial sketch. My "Sketch" category is a second sketch over the thumbnail, improving the quality and making it look much much cleaner.
I only color "Sketches", never thumbnails. http://i.imgur.com/C6XWHbW.png is an example of thumbnail quality (I turned nystre into a goat :B), although that thumbnail in particular has some bad anatomy-- the LINE ROUGHNESS dictates if it's a thumbnail or a sketch. I am unsure if I should even offer it.
I just wanted this difference cleared up since it's listed as an option. If you're confused or need more examples, please ask.
Link
Sylphy
Thanks! I was wondering what the term for an initial sketch was =3
A lot of artists leave it out of progress shots, but I've always wanted to see what kind of thumbnail they start with before things get cleaned up.