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.

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: