Roger,
Since this piece is taken from a Don Trioani painting (let's not even go there!), I'd take a good close look at that to see where improvements can be made. It's common practice for artists to copy a famous work as a learning experience, so I'd go that route.
Also, since it's a flat, you have to pick a light source in order to determine where highlights and shadows will go. Looking at this piece, I'm not sure where the light is coming from. In addition, the value range is narrow so it still looks a little, well, flat. With a little more work I think you have a nice piece going here.
Pete