That is intentionally not a feature in Weaver Xtreme.
The ability to add an alternate image for mobile on Weaver II Pro worked for “smart mode” only, and it is not a responsive design. It is a special detect the device design and actually not very elegant.
Weaver Xtreme, and essentially ALL modern WordPress themes are responsive in their design. That means that the header image will automatically resize to fit mobile devices, and it should be unnecessary to use a special image.
That said, if you do want one image for desktop and one for mobile, you can use the Main Options > Header > Header Image > Image HTML replacement with [show_if] shortcodes like below (Show_if / hide_if shortcodes are detailed in the Add-Ons page
[show_if device='desktop'] html for the desktop header image [show_if]
[show_if device='mobile'] html for the desktop header image [show_if]