The ornamental design for a lightbox, as shown and described.
FIG. 1 is a top perspective view of a light box showing my new design, shown sitting in a base which is shown in broken lines and forms no part of the claimed design;
FIG. 2 is a top perspective view of the light box of FIG. 1, showing my new design;
FIG. 3 is a side view of the light box of FIG. 2, with all side views being identical; and,
FIG. 4 is a top view of the light box of FIGS. 1 and 2, with the bottom view being identical.