How to set proper image size and aspect ratio in featured slider on a mobile?

Hello, please help me with this issue. I have set featured slider on my homepage www.lukasvesely.com from 2 sites. On PC the background images work absolutely well but on mobile phone, the picture has a very bad resolution. I uploaded the images as a featured images of both sites in 1920*1080 p. Please, how to set the image for PC and mobile separately?