Martin Luther is remembered as a man of words, ninety five theses, a German Bible translation, a mountain of sermons and letters. It is easy to forget he needed a painter at all, and easier still to forget that the painter was a personal friend rather than a hired specialist brought in after the fact. Lucas Cranach the Elder knew Luther directly, stood as godfather to one of his children, and painted his portrait more than once across their overlapping careers in Wittenberg.
Law and Gospel is where that friendship did real doctrinal work rather than merely social work. Reformation teaching on justification, the claim that a person is saved by grace through faith rather than through earned merit, is a genuinely difficult idea to hold onto against centuries of inherited assumptions about penance and good works. Cranach's panel does not argue the point. It simply shows two outcomes side by side, one governed by the old covenant's demands and one governed by grace, and lets the visual contrast do what a sermon would otherwise have to build toward across an hour.
That a reformer whose whole movement prized scripture and plain preaching over image and ritual still turned to a painted panel to make his central point land says something about the limits of words alone, even for someone who used them as well as Luther did. The picture was not decoration for the doctrine. It was, for many who saw it, the doctrine's clearest available form.