HTML code for image to fit screen [smartphone particularly]

I need something very simple. We are using QR codes. I wan to post images on a webpage. Here is the image: [login to view URL] I need the index page so that on a mobile device the width is 100%. I can not figure it out for some reason. Should be a piece of cake for someone else. The end result should look like the attachment. Really looking to spend $20 on this. Thank you.