Background Image Dimensions

I think different sizes will work, but the one in the demo is 2200px by 1280px.

I have a background image (1200px 1200px) set to fixed and that works just fine for an average-sized monitor. On a tablet (or a phone) the background image scrolls along with a transparent overlay that I created. And it leaves an ugly grey color at the bottom of those pages that are long.

I have tried using a media query to set the background-size various ways but no luck.
Any ideas anyone?

Sorry, the site is https://www.pathwaysoffaith.com and the page is Stories from the Path. Please excuse placeholders, we are still in development.

On what page are you having the problem? Homepage and all others?

On https://www.pathwaysoffaith.com/My Path/ and https://www.pathwaysoffaith.com/Stories From the Path/. Both pages have posts which extend the transparent page.

Thank you.
Bob