Well, tbh in the end the decision is always left for the voters to decide. Personally, I think having each image in it's own thumbnail is fine because after all you won't likely be showing them side by side after the challenge anyway as far as your portfolio goes. Whatever shows your work off to the best advantage is what really matters.

I could be wrong here but I think if you just post it up with the ### WIP... message on the final NEW image you should be good. I'll have to check again but I don't think the vote is based on who made the most improvement anyway.

In the past when we had contests based on an image (eg. Clouds) you only had to have the original cloud shot in the wip thread not on the final map.