Change Background Image in Preset

Hello,

I would like to use my own image instead of the preset background image.
For example, for the We Love Design Layout, I would like to replace the background taxi image with my own.

I have tried to edit the row in Design -> Background image -> Select image

Now this will only affect the background image of the actual displayed background image. So confusing !!

I have tried to change the custom css:
.sow-slider-image-overlay sow-slider-image-cover {
background-image: url(https://mysite.com/wp-content/uploads/2016/07/banner.jpg);
}

But this also gives a Server Error.

Can someone help me make this simple change?

There should be an option for this, I assume everyone who wants to use the presets would want to replace those typical stock images.

Cheers