Don't screenshot in the ImageWidget test,

instead verify that the base64 makes it to the page and
assume that the browser knows how render it correctly.

IPython/html/tests/widgets/widget_image.js

@@ -1,65 +1,48
 // Test image class
 casper.notebook_test(function () {
     index = this.append_cell(
         'from IPython.html import widgets\n' + 
         'from IPython.display import display, clear_output\n' +
         'print("Success")');
     this.execute_cell_then(index);
 
     // Get the temporary directory that the test server is running in.
     var cwd = '';
     index = this.append_cell('!echo $(pwd)');
     this.execute_cell_then(index, function(index){
         cwd = this.get_output_cell(index).text.trim();
     });
 
-    test_jpg = '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';
-
-    // It has been verified that both of the following are the correct image.
-    // Unfortunately, SlimerJS and PhantomJS disagree on a byte by byte level.
-    test_results_phantom = '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';
-    test_results_slimer = '/9j/4AAQSkZJRgABAQAAAQABAAD/2wBDAAYEBQYFBAYGBQYHBwYIChAKCgkJChQODwwQFxQYGBcUFhYaHSUfGhsjHBYWICwgIyYnKSopGR8tMC0oMCUoKSj/2wBDAQcHBwoIChMKChMoGhYaKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCgoKCj/wAARCAAyADIDASIAAhEBAxEB/8QAHwAAAQUBAQEBAQEAAAAAAAAAAAECAwQFBgcICQoL/8QAtRAAAgEDAwIEAwUFBAQAAAF9AQIDAAQRBRIhMUEGE1FhByJxFDKBkaEII0KxwRVS0fAkM2JyggkKFhcYGRolJicoKSo0NTY3ODk6Q0RFRkdISUpTVFVWV1hZWmNkZWZnaGlqc3R1dnd4eXqDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ipqrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uHi4+Tl5ufo6erx8vP09fb3+Pn6/8QAHwEAAwEBAQEBAQEBAQAAAAAAAAECAwQFBgcICQoL/8QAtREAAgECBAQDBAcFBAQAAQJ3AAECAxEEBSExBhJBUQdhcRMiMoEIFEKRobHBCSMzUvAVYnLRChYkNOEl8RcYGRomJygpKjU2Nzg5OkNERUZHSElKU1RVVldYWVpjZGVmZ2hpanN0dXZ3eHl6goOEhYaHiImKkpOUlZaXmJmaoqOkpaanqKmqsrO0tba3uLm6wsPExcbHyMnK0tPU1dbX2Nna4uPk5ebn6Onq8vP09fb3+Pn6/9oADAMBAAIRAxEAPwDzqiiivjz+kQooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigD//2Q==';
+    var test_jpg = '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';
 
     var image_index = this.append_cell(
         'import base64\n' + 
         'data = base64.b64decode("' + test_jpg + '")\n' +
         'image = widgets.ImageWidget()\n' +
         'image.format = "jpeg"\n' +
         'image.value = data\n' +
         'image.width = "50px"\n' +
         'image.height = "50px"\n' +
         // Set css that will make the image render within the PhantomJS visible
         // window.  If we don't do this, the captured image will be black.
         'image.set_css({"background": "blue", "z-index": "9999", "position": "fixed", "top": "0px", "left": "0px"})\n' + 
         'display(image)\n' + 
         'image.add_class("my-test-image")\n' + 
         'print("Success")\n');
     this.execute_cell_then(image_index, function(index){
 
         this.test.assertEquals(this.get_output_cell(index).text, 'Success\n', 
             'Create image executed with correct output.');
 
         this.test.assert(this.cell_element_exists(index, 
             '.widget-area .widget-subarea'),
             'Widget subarea exists.');
 
-        this.test.assert(this.cell_element_exists(index, 
-            '.widget-area .widget-subarea img'),
-            'Image exists.');
-
-        // Capture a screenshot of the img element as a base64 string.
-        var fs = require('fs');
-        capture_filename = cwd + fs.separator + 'captured.jpg';
-        this.captureSelector(capture_filename, '.my-test-image');
-        var stream = fs.open(capture_filename, 'rb');
-        var captured = btoa(stream.read());
-        stream.close();
-        fs.remove(capture_filename);
-
-        // Uncomment line below to output captured image data to a text file.
-        // fs.write('./captured.txt', captured, 'w');
+        var img_sel = '.widget-area .widget-subarea img';
+        this.test.assert(this.cell_element_exists(index, img_sel), 'Image exists.');
 
-        var match = captured == test_results_phantom || captured == test_results_slimer;
-        this.test.assert(match, "Red image data displayed correctly.");
+        // Verify that the image's base64 data has made it into the DOM.
+        var img_src = this.cell_element_function(image_index, img_sel, 'attr', ['src']);
+        this.test.assert(img_src.indexOf(test_jpg) > -1, 'Image src data exists.');
     });    
 });
  No newline at end of file