Set an ajustable and right aligned image in background

Hi,
Here is my website : ypineault.com

If you have a relatively small-sized monitor, you will normaly see an image at your right. I would like this image to be adjustable to the viewer screen but I cannot use the cover; function, since it will zoom my image.

The problem there is that viewers with large monitors won’t see the image and those with mobile will see an extra-large image, or a small one centered in the middle of the content page (around the 3 or 4th post).

Is there a solution to keep my image and make it visible to all my viewers? I have visited a lot of forums and website and I couldn’t find anything that helped me. I also tried to make the image not visible to mobile users, but it didn’t worked with comomn given code.

Note that I use twenty thirteen theme and I have the same problem with my header image, which seems to be a comon issue. Also note that on mobile, my image works perfectly on my “contact” page (probably because it’s shorter).

Thanks a lot for your help,

Yann