Index: server/Slim/Web/Settings/Server/UserInterface.pm
===================================================================
--- server/Slim/Web/Settings/Server/UserInterface.pm	(revision 14050)
+++ server/Slim/Web/Settings/Server/UserInterface.pm	(working copy)
@@ -16,6 +16,23 @@
 
 my $prefs = preferences('server');
 
+$prefs->setValidate({
+	validator => sub {
+					if ($_[1] =~ /^%(.*?)(\..*?){0,1}$/) {
+				
+						if (defined $2) {
+				
+							my $suffix = $2;
+							return !grep(/^$suffix$/, qw(jpg gif png));
+				
+						} else {
+							return 1
+						};
+					}
+				}
+	}, 'coverArt',
+);
+
 sub name {
 	return Slim::Web::HTTP::protectName('INTERFACE_SETTINGS');
 }