Picture in a Picture

After re-reading your post, I realize that you may want something different.  Take a look at the Guided Edits>Fun Edits which may be more in line with what you want to do.  These will take you step-by-step through the process.

See if this helps you any:   https://hooktube.com/watch?v=FBsnbvGOwZk

It's fairly easy:

• open the two pictures in the Editor,
• make sure the Photo Bin is toggled open,
• select the background photo so it appears in the editing screen, and
• click and drag the words photo from the photo bin on top of the open photo. 
• This will create a new smart layer that you can resize or resposition by using the move tool.