Random Background Image positoning

I’m working on a html that displays random background image, that i got from this tutorial:
http://wsabstract.com/script/cut176.shtml

I included this in my html and tried to re-sizing it to background-size:contain, and i noticed that the image is only set in the top section of the page; i think this means that the content space given for the image is only so little.

i attched the html file i made

I’m not getting on why this is happening i assumed that the code would give the whole page space to contain the background image. As a result i’m having a hard time, positioning these images on my page.

Some advice or help would be really nice. Thanks

-Sam