Blurry Images on Mobile Devices due to big pixels

Hi. My images appear alright when viewed on desktop, but when viewed on mobile, the images become much bigger, resulting in a blurry image. I've sort of found the solution here https://managewp.com/responsive-images . I tried option 1, but it didn't seem to work. I looked at option two, but it sounds too difficult and I don't really know how write all that code. Would it be possible if you can help me write the code and I can insert it in via the simple CSS plugin ? Thanks !