Manual:$wgCopyrightIcon

Details
Set this to some HTML to override the rights icon with an arbitrary logo. This is the logo in the bottom left of the footer opposite the MediaWiki logo if using the Monobook skin.

For example:

Deprecation and removal
This setting has been deprecated in 1.17 and has been removed in MediaWiki 1.27. ['copyright']['copyright'] should be used instead.