Website looks stretched on other monitors

here is my site – hostilegamer.com

i’m currently running on a 1920×1080 resolution and the site looks fine, perfect proportions and i can see the full background image.

but when i was at school today on some of the smaller monitors my site looked stretched out and really out of proportion and i couldn’t see the background image. the add on the side was also stretched out.

On my brothers laptop too, you could only see part of the background image.