Keeping header image height constant for different sized windows
I was trying to check how my homepage behaves when I shrink the size of the window. My screen resolution is 2560 x 1440. I have a noticed that as I decrease the length of my window, my homepage header image shrinks both in length and breadth. Is there any way to tweak this so that the height of the image remains constant throughout, until it's less than a particular threshold where the separate header image for my phone ( I have it already) takes over? (Have a look at zizka.ca to understand what I mean. Although I know it's a Squarespace site)
Also, I know there is something in my css which decides when the menu will be shrinked to an accordion, depending on the screen width. I would like to increase the threshold, but I'm not sure which line does it. Could anyone help me out with it?