Finally, after capturing the screen or WebElement, users can do a pixel or size comparison of the two images. It is difficult to keep the bitmaps in sync from browser-to-browser, or mobile device-to-mobile device, but the method is here for argument's sake:

public enum RESULT { Matched, SizeMismatch, PixelMismatch }

/**
 * compareImage - method to compare 2 images
 * 
 * @param expFile - the expected file to compare
 * @param actFile - the actual file to compare
 * @return RESULT
 * @throws Exception
 */
public static RESULT compareImage(String expFile,
                                  String actFile)
                                  throws Exception {

    RESULT compareResult = null;
    Image baseImage = Toolkit.getDefaultToolkit().getImage(expFile);
    Image actualImage = Toolkit.getDefaultToolkit().getImage(actFile);

    // get pixels of image
    PixelGrabber baseImageGrab =
    new PixelGrabber(baseImage,0,0,-1,-1,false);

    PixelGrabber actualImageGrab =
    new PixelGrabber(actualImage,0,0,-1,-1,false);

    int [] baseImageData = null;
    int [] actualImageData = null;

    // get pixels coordinates of base image
    if ( baseImageGrab.grabPixels() ) {
        int width = baseImageGrab.getWidth();
        int height = baseImageGrab.getHeight();
        baseImageData = new int[width * height];
        baseImageData = (int[])baseImageGrab.getPixels();
    }

    // get pixels coordinates of actual image
    if ( actualImageGrab.grabPixels() ) {
        int width = actualImageGrab.getWidth();
        int height = actualImageGrab.getHeight();
        actualImageData = new int[width * height];
        actualImageData = (int[])actualImageGrab.getPixels();
    }

    // test for size mismatch, then pixel mismatch
    if ( (baseImageGrab.getHeight() != actualImageGrab.getHeight()) ||
         (baseImageGrab.getWidth() != actualImageGrab.getWidth()) ) {
        compareResult = RESULT.SizeMismatch;
    }

    else if ( java.util.Arrays.equals(baseImageData,actualImageData) ) {
        compareResult = RESULT.Matched;
    }

    else {
        compareResult = RESULT.PixelMismatch;
    }

    return compareResult;
}