Create backgroundImageFillAndCenter

This html file contains the bare minimum to use the javascript code that gets the imaged (defined in the css part), resizes it to fill the screen, and displays its' center (if the screen is too small).
It works well on desktops and it is advised to use pictures optimized for web, as things can get intense if you try to load a 100mb picture on the browser.

NECESSARY IMPROVEMENTS: the code doesn't work very well with mobile platforms, because the window.onresize event gets triggered whenever the user scrolls down. A possible correction is to check if the screen really changed before calling the main function of the screen, avoiding false resizes. I also observed that on the iphone the image won't fill the entire screen, so this needs to be worked out aswell.