I understand that the image resizes according to screen-size and aspect ratio. However, I'm having trouble getting this to work across different types of devices. On a phone, it seems that portrait images work better, but on a computer, landscape works better. The result is that the compositions in my photos are spoilt because either the top and bottom are cropped-off in a computer, or the sides are cropped-off on a phone. 

I was wondering if it's possible to set up the 'classic menu' for the phone version, and the 'full-screen slider' for the computer version. If so, how can I do this? 

I've sorted this now with this plug-in: https://wordpress.org/plugins/equivalent-mobile-redirect/

It seems to work ok