use png for better transparency

core/src/main/java/hudson/model/ManageJenkinsAction.java

@@ -34,7 +34,7 @@ import hudson.Extension;
 public class ManageJenkinsAction implements RootAction {
     public String getIconFileName() {
         if (Hudson.getInstance().hasPermission(Hudson.ADMINISTER))
-            return "setting.gif";
+            return "setting.png";
         else
             return null;
     }

ui-samples-plugin/src/main/java/jenkins/plugins/ui_samples/Root.java

@@ -13,7 +13,7 @@ import java.util.List;
 @Extension
 public class Root implements RootAction {
     public String getIconFileName() {
-        return "gear.gif";
+        return "gear.png";
     }
 
     public String getDisplayName() {

ui-samples-plugin/src/main/java/jenkins/plugins/ui_samples/UISample.java

@@ -20,7 +20,7 @@ import static org.apache.commons.io.IOUtils.copy;
  */
 public abstract class UISample implements ExtensionPoint, Action, Describable<UISample> {
     public String getIconFileName() {
-        return "gear.gif";
+        return "gear.png";
     }
 
     public String getUrlName() {