The ornamental design for a lightbox, as shown and described.
FIG. 1 is a top perspective view of a first embodiment of a light box which embodies the present design shown in full lines; the items shown in broken lines are depicted for illustrative purposes only and form no part of the claimed design; only one side view of the light box is shown because the each of the side views are identical;
FIG. 2 is a side view of the light box of FIG. 1;
FIG. 3 is a top view of the light box of FIG. 1; and,
FIG. 4 is a bottom view of the light box of FIG. 1.