WP header images on mobile devices

I have just transferred my site http://guided-mountain-walks.com/ onto hosting server and main problem seems to be the header images on mobile devices are not responsive. I am using the WP header images plugin which puts specific images on individual pages but when I was in the development stage I had the same problem with the customized header.
Thanks for your which would be much appreciated

Seems to have resolved itself. I migrated it over to SSL - not sure if that’s a coincidence