Go to the page you want to add a parallax image to and press the EDIT PAGE button at top right.
Upload your image by clicking the INSERT IMAGE button above the wysiwyg editor. Make note of the exact filename you are using.
Once you do this, cancel out of the media manager popup box.
Place this code inside the content area of your page, where you want the parallax image to go:
[parallax img="church.jpg" height="400"]
where church.jpg is the filename you want to show there.
You can also play with the height number but it is good to leave those at what the code states above.
Click SAVE & VIEW at top right of page to view the parallax image.