Fixed background gets too big

Thank you once again for a beautiful theme. My website (personal) is soon finished. I’m having trouble with my background photos. They are constantly scaled too big, even though they are uploaded in correct ratio sizes. It makes it impossible to see what the background actually is and the pictures also gets blurry and “pixly” as they are blown up 200-400%.
I have tried adding (after trying to solve this myself) the custom css:
html {
height:100%;
background-size: cover;
}
But it has no effect.

What css code is needed to make sure that background photos looks better in fixed position? What’s your recommended picture size?

Thank you.