This is a limitation of Gracenote's API. It only returns album art when doing a SINGLE_BEST
lookup (called BEST_MATCH_ONLY
in the php-gracenote library).
You can pass an extra parameter into any of the search functions of php-gracenote to get album art, but you'll only get the single best result rather than multiple results.
// No album art, but multiple results.
$api->searchArtist("Moby");
// You get album art, but only one result.
$api->searchArtist("Moby", Gracenote\WebAPI\GracenoteWebAPI::BEST_MATCH_ONLY);