There are a number of different you can go for Reformation Sunday. These are some I am using:
The Door of the Castle Church in Wittenburg where Martin Luther posted the 95 Theses. The Theses are now engraved in the metal doors.
Lucas Cranach the Elder altarpiece in City Church of Wittenberg (1547)
Detail Left Panel
Detail top center panel
Detail Right Panel
Detail Bottom Panel
Martin Luther (1523) by Lucas Cranach