How to change body background color to image in blog template

If you're using standard blog templates, then you should see a block of CSS style. So what you need to do is go to Layout -> Edit HTML and look for something like this:

body{
    background: #3c3533;
    margin: 0;
    padding: 0;
    color: #fff;
    font-family: Verdana, Tahoma, Georgia;
    font-size: 12px;
    line-height: 20px;
}

then change background: #3c3533; to background: url(http://exmaple.jpg);
save the template and you're done.