Ladybug53 has raised a couple of good questions that other people may be wondering about, so I'm going to post the answers here.
1. Is it ok to use the Motherlode Cheat to set up props for a photo shoot? Yes. The Sim you'll be uploading to the contest is the one in the library, not the copy you're using to take screenshots, so if you want to give extra money to the copy of your Sim for screenshot purposes, that's fine.
2. Is it ok to use store content in the photo shoot (store furniture, etc)? Regretfully, the answer to this one is no. I know there are a lot of great store items that could be used to create a special effect in screenshots, but not everyone has access to store items, and we're trying to be fair to everyone.