As predicted, my next step was indeed to fill in the background and start working on the cast shadows of the marbles. This time the shading is deliberately painted that way rather than poor lighting and camera phone limitations. One of the bonuses of taking photos is that they help me see the painting as a whole. I am already finding areas that need more blending that I didn't notice before, even when I stepped way, way back on the piece. I am not done with the shadows by any means, and won't be until I start the marbles and can develop the relationship between the two. However, my goal in this step was to blend the outer edges of the shadows into the background and I can now see where I missed the mark. The shadow on the red marble in particular looks too hard. I also struggled with the shape of the shadow on the big blue marble and it shows. I guess I'm going back in...