I cloned out some of the chalk lines on the blackboard and moved the pear and duster over to the right. I used the source frog as reference to draw a rough chalk outline on the blackboard. I used the chalk brush in photoshop.
Copied and pasted the chalk on a new layer. Changed the chalk color to green.
Finished details on the frog.
Put some coloured chalk dust on the duster.
Added some yellow highlights on the pear.
Adjusted colors and contrast. Used the render lighting effects filter to put a spotlight on the frog drawing.