You need to think carefully about your requirements and what equal means. If you are doing a direct pixel by pixel comparison, then you can find numerous .net image classes to help with this (I am not a .net expert but try here Image Comparison for find percentage of similarity between images )
Of course implementing a simple direct image difference is fairly easy. You could even allow for slight differences with difference < threshold, which the tutorial in the above SO answer discusses. Incidentally this answer also mentions SIFT, which I did not realize until after I had mentioned it. SIFT is a good tool depending on your requirements.
The SIFT 3rd party tool is a great way to compare images that can have slight variations, but you might have to make a system call as I am not sure if they provide a .net interface. The definitive website for SIFT implementation is: http://www.cs.ubc.ca/~lowe/keypoints/
I did find this on the internet: http://www.nowozin.net/sebastian/tu-berlin-2006/libsift/ which claims to be a c# implementation.