Add the icon xminetype-png.

static/plugin_event/resources/css/kde-oxygen.css

@@ -13,3 +13,8 @@
      background-image: url(../icons/kde-oxygen/16x16/format-break-node.png);
      background-repeat: no-repeat;
 }
+
+.xminetype-png {
+     background-image: url(../icons/kde-oxygen/16x16/image-x-generic.png);
+     background-repeat: no-repeat;
+}

views/plugin_event/graph_mpl.html

@@ -62,11 +62,12 @@
     menu = Ext.create("Ext.menu.Menu", {
         items: [{
             text: "{{=trToPNG}}",
-            href: "{{=url_png}}"
+            href: "{{=url_png}}",
+            iconCls: "xminetype-png"
         }, {
             text: "{{=trToPDF}}",
             href: "{{=url_pdf}}",
-            iconCls: 'xminetype-pdf'
+            iconCls: "xminetype-pdf"
         }]
     });