Lingua Libre deserves a vector version for its top left logo. We can suggest to use https://commons.wikimedia.org/wiki/File:LinguaLibre_-_logo_SVG_-_two_colors.svg by Sukkoria.
This was historically impossible on MediaWiki, but since REL1_31 it should be now using $wgLogoHD (ref).
If not possible, use an higher definition png. The current one is easily pixelized.
I just said above that it should be possible now to use a SVG (and the current image is the biggest PNG we can have as a logo).