Hey @Tailwinds,
At this time, it's not possible to select different images for the mobile, and desktop version of your site. When previewing your site in a browser, you can toggle between mobile, and desktop view to make choosing a picture, and seeing what each option looks like a little easier. From your Square Online site editor, select Preview for these options to come up:
That being said I understand that some pictures might suit the desktop, or mobile version better - I'll pass this suggestion on to the Square Online team for consideration.
