Creating multiple Headers for Mobile formats Expound theme

This is my first time using wordpress, I am using the “expound” theme. Currently running off of my computer, so no link.
I would like to use my header image for mobile (ipad,iphone,etc) viewers as well.
If I use a child theme to create different size header images what sizes should I create and where do I include this code into the child theme?
Also would it be simpler to use a child theme to only crop from the right size of the header image?
Thank you.