I claim the ornamental design for a photo frame knob, as shown and described.
FIG. 1 is a perspective view of a photo frame knob, showing my new design;
FIG. 2 is an exploded perspective view thereof;
FIG. 3 is an exploded front elevational view thereof;
FIG. 4 is an exploded rear elevational view thereof; and,
FIG. 5 is an exploded right-side elevational view thereof, the left-side elevational view being a mirror image thereof.
The broken lines showing a cabinet in FIG. 1 depict environmental matter and form no part of the claimed design. The broken lines showing a leaf photo in FIGS. 1-3, and a circular element in FIG. 4, depict portions of the photo frame knob that form no part of the claimed design.