Presently, we do not support fixed background images. All row background images use the responsive CSS property of `background-size: cover` and the default scroll state for the `background-attachment` property. We do have an issue slated to add a control that would allow you to set `background-position` which allows you to center the focal point around which your image will responsively resize and we may be able work `background-attachment` in during that time. But unfortunately, there isn't a very clean way to do the CSS to override it as-of-today since none of the rows have unique classes on them and you should not use the dynamic ember IDs as those change.
I wanted to let you know that our latest release added the feature that you had asked about. You can now upload an image onto a row and then under the appearance settings for that image, set whether or not you want the background scrolling to be present. By default it is off and you would have what you have today, but if you turn it on, we will fix the background image (no CSS required) within the row and scroll content over top the image. Hope this makes things easier for you in the future!