1 Find the default background
The default background is stored in the subdirectory X-Plane 11 \ Resources \ bitmaps \ interface11. Browse this directory and search for the file image_background_screenshot_for_stack.png. Rename this file to image_background_screenshot_for_stack.OLD. If the file extensions are not visible due to local settings, it is also sufficient to simply add _old to the file name while renaming.
2 Insert your own background
Next, simply put your own image or screenshot to this subdirectory and rename it to image_background_screenshot_for_stack.png.
With the next start of X-Plane 11 you should see your own image.