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

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

+function takeScreenshot() {  
+  chrome.tabs.captureVisibleTab(null, function(img) {
+    var screenshotUrl = img;
+    var viewTabUrl = chrome.extension.getURL('screenshot.html');
+
+    chrome.tabs.create({url: viewTabUrl}, function(tab) {
+      var targetId = tab.id;
+
+      chrome.tabs.onUpdated.addListener(function(tabId, changedProps) {
+        if (tabId == targetId &&
+            changedProps.status == "complete") {
+          var views = chrome.self.getViews();
+          for (var i = 0; i < views.length; i++) {
+            var view = views[i];
+            if (view.location.href == viewTabUrl) {
+              view.setScreenshotUrl(screenshotUrl);
+            }
+          }
+        }
+      });
+    })
+  });
+};