Sheepster,
It's perfectly fine to replace the original image and just make a post like "I've updated such and such on page..." Or for bigger changes (total redraw or additional profiles) feel free to make a new post.
I don't think there are any hard and fast rules, people only tend to get annoyed if you spam dozens of reworkings every ten minutes!
Looking forward to seeing the update, I've a feeling we're going to see many more of Saunders Roe's designs!
Its good to see one of the smaller and often overlooked companies getting represented here.