Adaptive images creates various image sizes and delivers the best file size depending on the visitor’s device. Helps to reduce download time on mobile devices (I’ve seen this reduce download size by 50% on one site in mobile).
More Info on Adaptive Images
Settings > Adaptive Images
Default settings work well, but enable the option HiDPI support.
Automatic, but if you need to force it to deliver the original image in all devices, append
to the end of the image source URL like in this example: