fastest way to compare 2 bitmaps

You can use the LockBits and UnlockBits methods to get the image data from a bitmap:

Get data:
 
Dim bounds As New Rectangle(0, 0, bitmap.Width, bitmap.Height)
bitmapData = bitmap.LockBits(bounds, ImageLockMode.Read, PixelFormat.Format32bppRgb)
 
Dim size As Integer = bitmapData.Stride * bitmapData.Height
Dim imageBytes As new Byte(size)
Marshal.Copy(bitmapData.Scan0, imageBytes, 0, size)
bitmap.UnlockBits(bitmapData)

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The fastest way is to compare internal bitmap structure, by taking the HBITMAP handle of each.
It is extremely complex, have a look to the C++ code:
http://www.codeproject.com/useritems/BitmapCompare.asp

Another interesting debate is present here from
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=1057994&SiteID=1

If important thing is not the pixels to be equals, just human readable eye difference, then jo0ls
has given good argument