To create full screen image use css:
html {
background: #e7e9ef url('image.jpg') no-repeat center center fixed;
-webkit-background-size: 100%;
-moz-background-size: 100%;
-o-background-size: 100%;
background-size: 100%;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
min-height: 100%;
}
body {
min-height: 100%;
margin: 0;
padding: 0;
}
Your image not have to be full image screen resolution - it will be stretched but of course to achieve decent effect you should rather use quite big image - I used 1920px x 1285px but of course it need to be compressed as much as possible.
You should also consider what happened if user visit website using mobile devices. Big images takes a lot of transfer so you should also consider preparing smaller images and use media queries to give such user smaller image.