Mac and Windows will 2x all images for users with HiDpi displays. You can use CSS to display the images as background-image then media queries are available to serve differently sized images for HiDPI. Alternatively, most images look completely fine scaled down, they just get blurry scaled up. If you use an image that is 2x bigger than defined in the img tag it will play nice on HiDPI.

I cannot vouch for this one, but Apple has a suggestion https://developer.apple.com/library/safari/documentation/Net...

Use the srcset attribute to provide @2x sizes.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/Im...

Example: <img src="img.jpg" srcset="img@2x.jpg 2x" alt="">