The ornamental design for a plate for supporting a picture, as shown and described.
The single view is a front view of a plate for supporting a picture showing my new design.
The rear view of the plate for supporting a picture is a mirror image of the front view shown in FIG. 1.
The left and side views, and top and bottom views, are devoid of surface ornamentation. Portions of the article not shown in the drawing form no part of the claimed design, and are disclaimed.