This feature only works for paid subscribers.
Many images have far too much constrast to be effective as use for a background. There are full whites and full blacks. Regardless of the color of the text, something is going to be unreadable.
So you either need to prepare your image so that this isn't a problem, or use this checkbox to change the entire image so that it is "washed out", or much closer to white than the original image.
In general, it's best if you do the preparation on your own computer to make the image "just right" before you send it.