These are original production drawings which are made by the animators on the show. After a drawing was completed it was then transfered to a clear sheet of acetate by basically using a special kind of xerox machine. The acetate with the now transfered drawing on it was then giving to another animator who paints in the reverse side of the cel image. After he's done you have your animation cel.
8.74/10 
9.21/10
9.03/10 
51 votes