CSS Background Image not displaying on mobile
So, what are some other differences between those two images? Mostly pertaining to pixel size.
Mobile Safari has a pixel threshold, which actually has nothing to do with size in kb of the image, but the amount of pixels. What are the sizes of the images that aren't displaying? Are they longer or taller than the ones that are?
Here is a link to the Safari Web Content Guide. Scroll down to the Known iOS Resource limits and see if you fall under any of those categories!
This 2014 question is apparently still relevant in 2021. I just had an issue of a full page background image not loading on a OnePlus 5 when it loaded fine on every other device. Resized the image to 1400px width as per above comment from SlightlyClever, made that variant the one shown on mobile version, and it loads up fine. At the same time cut down the download from ~3MB to a bit under 1MB so that's nice too.