Fix the location of blank.gif in OverviewMap.js. Closes #542.

git-svn-id: http://svn.openlayers.org/trunk/openlayers@2822 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Control/OverviewMap.js

@@ -111,7 +111,7 @@ OpenLayers.Control.OverviewMap.prototype =
         this.extentRectangle.style.overflow = 'hidden';
         this.extentRectangle.style.backgroundImage = 'url(' +
                                         OpenLayers.Util.getImagesLocation() +
-                                        '/blank.png)';
+                                        'blank.gif)';
         this.extentRectangle.className = this.displayClass+'ExtentRectangle';
         this.mapDiv.appendChild(this.extentRectangle);