I think your CSS is OK, (I am not an expert at CSS but I am currently learning it myself).
I think you issue is a cerficate error on the page you are hosting
try this:
body {background-image: url("https://image.shutterstock.com/image-vector/sample-stamp-grunge-texture-vector-600w-1389188336.jpg");
background-size:cover;}
I tested it in the additional CSS on my page and it worked.
I also added it to my stylesheet in my child theme and that also worked.
Hope that helps!
Cheers.