Cover Images poor quality on mobile

I have numerous cover images throughout my site and they come out crisp and clear on desktop and tablet but are notably of lower quality when they render on mobile. To make things more interesting, if I turn my phone on its side so as to go into landscape mode the images will instantly sharpen up and stay sharp when I switch back to holding the phone vertically again. However, if I then go and refresh the page it is the same lower quality image that is rendered and I must repeat the process of turning the phone on it’s side in order to get the high-resolution image.

Why is this happening and what can I do to fix it????