> +    int size = 16656;

>  Why this size? Is it reproducible with smaller buffer?
This just happens to the size of the image I was embedding into a html file.
I had two other images that failure as well and from memory they where bigger.
As a test, I subtracted 2 from all the value returned in CryptBinaryToStringW
and nothing failed.  So, I'm guessing a smaller size would cause the issue as well.

>I think this should test actual output, matching length alone is not enough.
Sure, however I don't have a problem with the data itself just the size returned.

Best Regards
 Alistair.