Depending on which theme you're using, you would go into the css file, find the line that says background, which should be near the very top, and change it to this:
PHP Code:
background: url("PutDirectImageLinkHere") no-repeat;
background-size:100%;
If you're using the default/main theme, this would be found in templates>main>media>style-city.css
The background size makes sure the image fills the screen without any white space and the no-repeat makes sure the image doesn't show up in tiles.