How to adjust image size for mobile on REFRESH theme?

Just started using shopify and cant seem to figure out how to make images look the same on desktop and in mobile. Im using the REFRESH theme. Thanks in advance.

can you please share scrrenshot that explain the problem? or share your store URL

almahaven.com, ive seen other pages in which the banner doesnt get stretch or shrunk depending on the device. The image stays the same as you placed it on the editor. In the images I added you can see that even as how small I make the page the image stays the same, thats what I would like.