Functionality has been requested in the Extension API for Javascript to...

chrome/test/data/extensions/samples/screenshot/functions.js

+// Copyright (c) 2009 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var button = null;
+var my_window = null;
+var screenshot_url = null;
+var child_loaded = false;
+var have_screenshot = false;
+
+function registerHandlers() {
+  button = document.getElementById("button");
+  button.addEventListener("click",handleClick);
+}
+
+function waitForChildLoaded() {
+  if(child_loaded && have_screenshot) {
+    var foo = my_window.document.getElementById("replacer");
+    foo.src = screenshot_url;
+    have_screenshot = false;
+  } else {
+    window.setTimeout(waitForChildLoaded, 50);
+  }
+}
+
+function handleClick() {
+  child_loaded = false;
+  chrome.tabs.getVisibleTabCapture(null,
+  function(img) {
+    try {
+      have_screenshot = true;
+      screenshot_url = img;
+    } catch(e) {
+      for(var f in e) {
+        console.log(f + ":" + e[f]);
+      }
+    }
+  });
+  my_window = window.open('popup.html');
+  window.setTimeout(waitForChildLoaded, 50);
+}
+
+function childLoaded() {
+  child_loaded = true;
+}