Why are my button images blurry on mobile screens?

I have a few custom buttons I’ve inserted with php throughout my wordpress site. They are crisp resolution images (either gif or png), but when viewing it on my iphone screen, they’re blurry. I noticed the other plugins I have installed that insert the same type of .png images buttons look very sharp when viewed on a mobile screen, or any screen for that matter.

What am I missing? Can’t figure out why a png inserted button from a wordpress plugin looks crisp on all screens, but my png buttons only look good on desktop screens.