Manual:$wgLogo

Details
The URL of the site logo. Either a fully-qualified URL or a relative path from DOCUMENT_ROOT. The logo is expected to be 135 x 135 pixels, but this can be increased by modifying the site CSS (and smaller sizes are possible with no changes).

If it's a relative path, it has to start with a slash (a file in DOCUMENT_ROOT is /file.png, not file.png).

Do not simply overwrite the default logo installed with MediaWiki ; this file will be overwritten when you upgrade.