Hi, Do you mean as Background image whole Helix template. If yes, in current version of helix - you have to use custom CSS and set background image for body tag. No WebP format support yet. Anyway very easy, beginner level.

body { 
 background-image: url("images/background.webp"); /* The image used */
  background-color: #cccccc; /* Used before image would be loaded */
  background-position: center; /* Center the image */
  background-repeat: no-repeat; /* Do not repeat the image */
  background-size: cover; /* Resize the background image to cover the entire container */
}