I found the solution ( $wgExtensionAssetsPath )
How to contruct the background-image filename from a value in one of the OpenID PHP modules ?
For a single rule, you can get away with something like:
$wgHooks[ 'BeforePageDisplay' ][ ] = function ( &$out, &$skin ) { $out->addHeadItem( 'openidstyle', '<style>#openid_provider_Google_icon { background-image: url(' . $wgExtensionAssetsPath .'/OpenID/skin/icons/Google_large.png); } </style>' ); return true; };