Edit the alignment on a background image for desktop and mobile

My client wants to add three different graphics as background throughout their website. I have noticed that Squarespace automatically centers background images and crops the sides as you move into mobile devices. Is it possible to: a) edit the size of the graphics, b) have different alignments for each graphic depending on the section, and c) edit both the size and alignment of these graphics for mobile?

We can use CSS code to add images to these position. If you send 3 images + site url, we can check easier