Placing a fixed image at the top of a page that works across desktop, tablet, and mobile.

I am SO confused. New to WP and using the free Astra theme. I picked a template that I felt suited my business (I do fairy hair, feather extensions, and glitter tattoos). In the template there was a neat feature of a fixed image at the top that the content scrolled over. I somehow figured out how to swap out that image with my own, but I tried so many things that I can’t quite figure out how I did it. On one page, the image looks fine on a browser, but when I view it from a mobile device, the original stock image appears. I can’t figure out what it is. I don’t think it’s using a bg image, because when I changed that in global settings it didn’t look the same. I kind of think it is using featured image, but I’m not sure, and looking at the code did not help me. Right now I’m trying to make a template page that I can copy for making additional pages. I’m trying to replicate this fixed image, and having it work on desktop, tablet, and mobile. Can anyone guide me? main page here:


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