The ornamental design for semi-panel image for a cap, as shown and described.
FIG. 1 is a rear perspective view of semi-panel image for a cap showing my new design;
FIG. 2 is a rear elevational view of FIG. 1;
FIG. 3 is a right side elevational view of FIG. 1: the left side is a mirror image; and,
FIG. 4 is a top plan view of FIG. 1.
The broken line showing of the visor is for illustrative purposes only and forms no part of the claimed design, and the broken line shown on the crown represents stitching.