Improve image positioning configuration

[sthuber90]

At the moment an image always fills the screen. It should be configurable to cover the background or fit in the image into the background i.e. portrait pictures in landscape mode should fit in window and leave black bars on each side. Also, panorama pictures never actually fit in the screen, so maybe the aspect ratio can be used to fit the pictures into the screen

[sthuber90]

Ok, after checking it's already possible to set background-size CSS property to cover or contains. Maybe I need to take the aspect ratio into account and change the background-size property depending on it