Ok...so,I'll call this a shadow.An opaque one.

Apps you need:
-a render
-photolayer
-background{optional}

Step 1:
Go to photolayer.Crop your background to whatever size you need.Then,press "add photo".

Crop the photo however you like.
You will see this after you have added the photo.

Click shadow.

You will then see this.

Adjust it however you like.

Then click save.

That's all.:DHave fun!

~Admin Peanut