There is no API or a way for the WPF testing framework to obtain the real bitmap out of the image source. However you can eventually read the bitmap manually as you do in WPF by obtaining first the image source. See this feedback
on the matter. Please note however the API has been updated an year ago so you will need a newer version of the product to use the extension we've added.
I would however look into a different approach - the built-in Image Verification feature
in Test Studio. It's a straight-forward image comparison solution which you can give a try as well.
Telerik by Progress