How do I keep background images in multiple sections on one page the same ratio?

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Hi y'all, I am trying to figure out how to get the background image of my sections appear the same in the sections that they are in. If you scroll on the page, you can see that some sections have a light overlay of sorts. Is there a way for me to make the background images the same ratio in each section so that the pattern doesn't appear bigger or smaller depending in the section they're in? For example, this is the first section the background is in:

and this is the last section the background is in:


OR, is it possible for a background to span the entire page (providing all of the content is moved to one section) without the background appearing pixelated? I imagine there are specific dimensions that make this possible, or coding that prevents images from stretching, but I haven't been able to figure it out, despite browsing numerous guides. 

Thank you!

Hi, commenting to boost this post. I still haven't figured this out and would appreciate any help.