Manual:$wgLogoHD/en

Details
Array with factors and their associated relative paths or URL paths to HD versions of the wiki logo. The scaled logo size should be under 135x135 pixels, the same as the size should be for the file of.

Only 1.5x and 2x versions are supported. That means e.g. the 2x version should be a square and should maximally have 270x270 pixels.

Example:

Older MediaWiki versions
If you're using MediaWiki versions older than 1.25, you can add the following snippet to your MediaWiki:Common.css: