I have submitted my final image to bigstockphoto.com and am currently waiting for it to be approved/rejected. I think it will most likely be approved because i've met all the requirements and I can't see any major problems with it.
I chose this sort of design as my final idea because after doing research on bigstockphoto, the most popular sorts of images seems to be the simple ones. In many of the searches I made on the site, the most popular were photos of a full moon, or a water droplet for example, so I decided to combine the two together into one, so technically if it was sold, the buyer could choose to crop out either aspect of the image (though unlikely).
I think the final image turned out pretty much exactly how I wanted it to, it was a good combination of the three sketches I did initially, and i'm happy with the quality of the photos. Also, I think i'm satisfied that my first two initial ideas gave me the ideas I needed for the final piece, without them it would have taken a lot longer to get an idea for what to do.