Manual:$wgResponsiveImages

Details
Whether or not to output high-resolution images for high-DPI displays.

This is primarily targeted at mobile and tablet browsers and some newer laptops such as the 'Retina' MacBook Pro. On these high-DPI devices, raster images appear blurry or pixelated by default without special treatment.

When generating thumbnail images, thumbnails are rendered not only at the requested size, but at 1.5x and 2x the requested size. The HTML5  attribute is included on the   specifying these larger images for the appropriate display densities.

WebKit has added native srcset support as of August 2013, but this will take some time to roll out to released browsers.

For current browsers, compatibility JavaScript is loaded which checks  for most browsers, and a compatibility fallback for IE 10, and applies srcset -> src change automatically. On browsers with native  support, this disables itself automatically.

Known limitations
Currently (as of 1.21) this code applies only to images created through Linker methods; image galleries and file: page detail displays are not currently affected. This should be changed at some point by migrating support lower down in the MediaHandler arena.

Display densities
Currently (as of 1.21) the 1.5x and 2x densities are hardcoded and not configurable. 1x is the implied default density of the main image.