no problem visible in your site;
this is what the theme outputs as #header
style into the <head>
section:
div#header {
background: url(https://schememagazine.com/wp-content/uploads/2012/07/Scheme-Magazine-Logo-black.jpg); height :230px; -moz-border-radius-topleft:6px;border-top-left-radius:6px;-moz-border-radius-topright:6px;border-top-right-radius:6px;
}
there is no ‘no-repeat’ in the background style; i.e. any background image will be tiled/repeated to fill the space automatically.
edit functions.php;
find:
div#header {
background: url(<?php header_image(); ?>); height :230px; -moz-border-radius-topleft:6px;border-top-left-radius:6px;-moz-border-radius-topright:6px;border-top-right-radius:6px;
change to:
div#header {
background: url(<?php header_image(); ?>) no-repeat; height :230px; -moz-border-radius-topleft:6px;border-top-left-radius:6px;-moz-border-radius-topright:6px;border-top-right-radius:6px;
for more background parameters such as background position, see https://www.w3schools.com/css/css_background.asp