The ornamental design for the skull backpack, as shown and described.
FIG. 1 is a front-right perspective view of a skull backpack showing my design;
FIG. 2 is front-left perspective view of the skull backpack shown in FIG. 1;
FIG. 3 is a backside perspective view of the skull backpack shown in FIG. 1;
FIG. 4 is a front-top perspective view of the skull backpack shown in FIG. 1;
FIG. 5 is a back-top perspective view of the skull backpack shown in FIG. 1;
FIG. 6 is an upper-left perspective view of the skull backpack shown in FIG. 1;
FIG. 7 is a lower-left perspective view of skull backpack shown in FIG. 1;
FIG. 8 is a front-left perspective view of the skull backpack shown in FIG. 1, with the skull backpack semi-open; and,
FIG. 9 is a front-left perspective view of the skull backpack shown in FIG. 1, with the skull backpack fully open.
The dash dot dash lines in the figures are for the purpose of illustrating the claim boundary and form no part of the claim. The evenly dashed broken lines showing a zipper and portions of a backpack are for the purposes of illustrating portions of the article that form no part of the claim.