How do I make my homepage responsive to mobile devices?

When I am on desktop my homepage background photo is in full screen. Once I choose mobile viewport the photo ratio stays the same so that half of the screen is white and half is the photo.

How do I make photo fill the screen, but yet not be larger than the screen-size?

This is my current CSS

.filldiv {
  /* height: 100vh; */
  max-width: 100%;
  height: 100;
  width: 100;
  margin: 0;
  padding: 0;
  box-sizing: border-box;
  opacity: 80%;
  filter: blur(8px);
}