How can I put an image behind an object in the background?

[gods10rules]

I'm trying to figure out how to but the mermaid behind an object in the background image. I've tried watching YouTube videos, but can't really comprehend them, and they all seem like too much work for something that should be simple.

These two pics are separate layers, there should be and easier way to put an inserted picture, behind an object in the background

For example, I'm trying to put the mermaid behind the chair facing the wall, but a part of the tail is still covering the chair.