The SVG cels have a coordinate system determined by the creator of the resource (which is part of the design of the SVG standard). PNG and JPG resources are refered to by pixel coordinates, with the lowest, leftmost pixel refered to as (0,0) and the uppermost, rightmost pixel refered to as (w-1,h-1). Note that these are ``traditional'' coordinates, not ``computer graphic'' coordinates, and that they are zero-based indexes. In practice, however, this is of little concern to the designer. For both types of cel, the designer defines a ``characteristic scale dimension'' (d) which is used for scaling the cel as it is placed. This should be something obvious to the designer -- for example, it could be the height of an animated character.
Each cel also has a ``pivot''. This is the pixel (or coordinate) which is used as the placement point for the cel. It is also the point about which the cel will be rotated. This would typically be the point right between the feet for a standing character or the center-of-gravity for a floating character, for example.