Is it safe to keep a picture or polaroid behind an iPhone?

The photo might discolor or degrade the back of the iPhone, but probably won’t eat a hole through the back itself.

The heat may or will degrade the photo, as well. Polaroids don’t do well warm.

If you’re not fussed about the back case getting discolored, and not fussed about the photo getting damaged, sure, have at.

There are vendors that offer a custom case, too: https://www.shutterfly.com/t/custom-iphone-cases/

Putting a picture between the phone and a clear case will not damage the phone.

You really need to clarify this.

As noted, your question does not make any sense. What are you doing or want to do?