use PNG

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

@@ -511,9 +511,9 @@ public /*transient*/ abstract class Computer extends Actionable implements Acces
     @Exported
     public String getIcon() {
         if(isOffline())
-            return "computer-x.gif";
+            return "computer-x.png";
         else
-            return "computer.gif";
+            return "computer.png";
     }
 
     public String getIconAltText() {

core/src/main/resources/hudson/model/ComputerSet/index.jelly

@@ -66,7 +66,7 @@ THE SOFTWARE.
           <td><!-- config link -->
             <j:if test="${c.hasPermission(c.CONFIGURE)}">
               <a href="${rootURL}/${c.url}configure">
-                <img src="${imagesURL}/32x32/setting.gif"
+                <img src="${imagesURL}/32x32/setting.png"
                      title="${%Configure}" alt="${%Configure}"
                      border="0"/>
               </a>