Trying to make image full width and responsive

I added some CSS that I found suggested online to allow my image to go full width and ignore the margins that were set up for the body content in the theme I’m using. However, once I did that the image I’ve put in isn’t responsive.

I’m not super familiar with coding CSS, so I would appreciate any help. Here’s the site where you can see my image: https://basiccon.org/register

And here’s the CSS code I added:

html, body {
  height: 100%;
  width: 100%;
  padding: 0;
  margin: 0;
}

#full-screen-image {
  min-height: 100%;
  min-width: 1024px;
  width: 100%;
  height: auto;
  position: static;
  padding-top: 40px;
  left: 0;
  margin-left:-34%;
}

#wrapper {
  position: relative;
  width: 800px;
  min-height: 400px;
  margin: 100px auto;
  color: #333;
}