The background of Snow theme it's already an image, but it's a small one that gets repeated for the entire screen. The easiest way would be to remove this image and upload your own image with the same name as the current one. You can change this image by accessing:
Snow Theme > images > bodybg.jpg
And then you need to remove the CSS style that repeats the background, in case you don't want your new image to be repeated for the entire page. You can do this by accessing:
Snow Theme > qa-styles.css
Open this file with Notepad++ or your preferred text editor. Around line 79 you'll see something like this:
body {
background: #eeeeee url(images/bodybg.jpg) repeat left top;
text-align: center;
margin: 0 0 80px;
padding: 0;
}
On the line that says:
background: #eeeeee url(images/bodybg.jpg) repeat left top;
change it for something this for example:
background: #eeeeee url(images/bodybg.jpg) no-repeat center;
You can learn more about the CSS' background property at: http://www.w3schools.com/cssref/pr_background-attachment.asp
