I'd say what you've got is fine so far. Eight tiles plus a larger image, first draft complete. I don't think the mockup needs to make much sense because you're not doing a full tile set, just enough to show that these few pieces can fit together. Looking rather good, actually.
It's your choice if you'd rather make a full tile set, but if you'd rather follow through with the challenge then complete these tiles instead. Polish them up until you think they are good enough to get paid for, and check the times.
If you want data to revise and add to what you have, maybe use one of the object/treasure generators to add a relic or two. The Serendipity site has a treasure generator (
http://nine.frenchboys.net/treasure.php) that produces results like:
Lying here is a statue made of rosewood, depicting a series of interlocked circles.
You unearth a stone plaque depicting a battle. It is unnaturally cold to the touch.
You have found a candleholder made of battered steel.
Mostly fantasy stuff, so you' might need to adapt it for your realm. A sci-fi light lamp instead of a candle holder, for instance.
Finishing up these tiles up is good enough to say challenge complete without the extra rework though. The after-challenge activity is to figure if this was a reasonable simulation of your real work flow, figuring out what was important to getting the tiles done (like the sketching you mentioned), and maybe revisit the original request that started the thread as if it were a sample job.
Tourist