Portrait format for header of mobile phones

Hi, I'm currently updating my webpage and would like to change to the Bold Photography (maybe Pro version) Theme. It would be essential for me as photographer that I can choose different photos for the header slider for the desktop version than for the mobile version (to prevent that just the center of the desktop is shown). I would like to choose some photos in portrait format and show them as the header slider on the front page for mobiles. Is there an (easy) way to adjust for this? Here is the link to my work-in-progress page: http://staging.flurinleugger.ch Thanks a lot for your help! Flurin