MediaWiki Manual $wgResponsiveImages
www.mediawiki.org/wiki/Manual:%24wgResponsiveImages
What Jamie wrote
I went searching to see what plans if any MediaWiki had for High-DPI images. Low and behold, 1.21 (not released) adds a setting for this! Nice!
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.