So the artwork is almost finished! I thought I'd take a minute here to visually explain how I use the low-res sketch as a "onion skin" layer as a rough guide for placement of elements in the final.
Below is a sample of what the enlarged low-res art looks like at 100% opacity and 100% actual size on my screen, once I place it in the hi-res, larger file...
...but during my work, I just use that layer as a transparency, as a rough guide under the working layers.
As you can see on the below picture, I create all of the details as I go along, on the fly. I already have an idea of what I want to do, but a lot of it is actual painting and you can plan some of that, but for the rest of it, just go with the flow. At least I do. That is more exciting. (What fun is planning every single pixel?)
Here are more examples of details in the final puzzle art at 100% working size on my screen:
And here is the final entire illustration..
...and then, Ta da! Here is the artwork in puzzle form.