or
System.Drawing.Bitmap expectedbmp = new System.Drawing.Bitmap("myExpected.png");
HtmlImage img = Find.ById<HtmlImage>("myImageId");
System.Drawing.Bitmap actualbmp = img.Capture();
ArtOfTest.Common.PixelMap expected = ArtOfTest.Common.PixelMap.FromBitmap(expectedbmp);
ArtOfTest.Common.PixelMap actual = ArtOfTest.Common.PixelMap.FromBitmap(actualbmp);
Assert.IsTrue(expected.Compare(actual, 5.0));