Here's my css for my background image (the clouds loop) :
background: url('https://up.l3m.in/file/1606821258-clouds-optimized.gif') no-repeat 100% 100% fixed;
The cool little tricks are the size of the bg image (100% width and 100% height), the fact that the image is fixed (it will move if you scroll on the page), and the background-size: cover does a zoom of the image if it is not to the right proportions.
The fact that I set the background in a pseudo-element forced me to define the size of this element (content, top, left, right & bottom properties).
Here are some links if you're interested:
Have a nice day trying things in css :)