Image degradation when used as background


I have built a page with an image via url and it looks nice and sharp.
I have a folder on the server with my images well prepared and optimised for size and quality.

I’ve then used the same image as a background to a block.
I couldn’t see how to reference it as a url so I added it to the project assets.
It looks awful. Very low quality.

Does Blocs try to optimise image assets?
Is there a block or brick that can use an image referenced by url as a background?

I come from using RapidWeaver and Stacks.
My main business site has hundreds of images so I reference them from url’s to keep the project size down (plus a myriad ion other benefits).