Hierarchy of Symbolic Representation?
Although no specific research establishing a hierarchy of symbolic representations could be identified, a review of the literature points to the following hierarchy as a possibility:

Gestures with an object – in context
Examples:
During snack, holding an empty cup out to an adult with a milk carton to request something to drink.
Pushing a plate with vegetables away to reject something.
While playing with blocks, holds a block out to the adult to request help to build.
After storytime, gives a book to an adult to request read.

Gestures without an object
Examples:
Points to objects to request.
Holds hand out with palm up to request what other person has.

Photographs
In addition to a photograph, another technique is to cut out around the object which will give the shape information, usually referred to as TOBIs (True Object Based Icons).

Detailed graphic
This would be similar to detailed clipart or cartoon drawing.

Line Drawing
Stick figures

Objects
scale model to represent larger object

Signs
Formal sign language

Words
Uses verbal words to communicate meaning

Keep in mind that skills will overlap and not be replaced by preceding skills.