I like to leave helpful information here for anyone else who is just starting out.

The in game screenshot(s) that are needed for a submission are referenced here:

https://www.thesimsresource.com/content/view-post/post/22938/Screenshot%20Requirements%20%284%3A3%29

The real requirements are a little more complicated than that, and could probably almost be case by case depending on the item type being submitted. To put it as easily as possible, the screenshot needs to obviously reflect which subject in the screenshot is the item being submitted. For example, I made a wall art portrait of a sim, and put the sim in the screenshot with the item. It seems like a good way to put a personal item like that on display, but this is too confusing at first glance and could be misread as having uploaded a sim instead of what it was for, the wall art.

I hope this might help somebody else :)