Another Luther Legend?

Tons of apologists and polemicists alike -- both Catholic and Protestant -- point to Luther's image of a snow-covered dunghill to illustrate the Protestant notion of imputed justification.

The problem is, it appears that the source for this image is dubious at best. This observation first came to my attention when I read Bill Cork's article Justification By Faith: Can Catholics and Lutherans Agree? Here is the relevant section from Bill's paper:

As I've already said, Sungenis dismisses Luther's understanding of Justification by referring to the "snow on dung" canard. Funny thing about that quote—despite years of Lutheran seminary education, and experience as a pastor, and the reading of umpteen volumes of Luther's Works, I have never come across that quote in print, nor have I ever heard it from the mouth of a Lutheran. Yet it is a favorite of Luther's critics [and, I might add, Protestants, e.g. James While in his The God Who Justifies]. I asked renowned Luther scholar Eric Gritsch about this, and he replied that it does exist somewhere in one of the "Table Talks" (after dinner ramblings written down by Luther's students—not reliable sources for Luther's thought), but even he couldn't give me a reference.

Bill's observation is corroborated by an answer given to a question at the Wisconsin Evangelical Lutheran Synod website:

With regard to the Luther quotation [the snow/dunghill image], a check in the indexes of four major editions of Luther's works does not reveal whether or where Luther might have said or written that.

While the image may faithfully represent the notion of justification held by modern Protestants, it seems unlikely that Luther used it, and also that it accurately reflects his own doctrine of justification.

If anyone has any detailed knowledge of where or how Luther uses this image, please let me know via comments or email. What seems clear is that Luther never used it in his written works or lectures; while it may appear in Table Talk (and I await a citation on that), those discussions most certainly hold a lower place than the other works of Luther.