I’ve been having an issue with the resolution of my portfolio banners on my phone. As you can see below, the resolution of the banners are a lot higher on my PC compared to my phone.
Phone: Samsung Galaxy S8
Banner resolution: 500 x 100
Your computer screen is much, much bigger than your mobile screen. Thus, the images appear smaller, in contrast to the large screen size. A smaller image will be a lot more crisp - take this game icon for example (below). At a small size, it appears like this:
However, scaled closer, you get:
The pixels are much, much more noticeable, as you are closer to them (they are enlarged).
In this same way, mobile images are scaled upward, to fit the screen, thus displaying the pixels more, making it seem more low resolution.
Personally, I cannot notice the difference between your mobile image and computer image. Both look great, they won’t throw anyone off. It has to do with scaling, screen space, and pixel size. You really can’t fix that - it shouldn’t hinder anything 
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