Sub Theme are made of options preset AND Custom CSS.
You have a rule
html {background: url(/wp-content/themes/weaver-xtreme/assets/images/desert.jpg) no-repeat center center fixed; -webkit-background-size:cover;
-moz-background-size:cover;-o-background-size:cover;background-size:cover;}
applying the background image to the html tag.
It seems to be in Main Options > Wrapping Areas > Background Image section, either with the image URL in one of the background boxes or the full CSS rule in a background CSS boxes.
remove it