The ornamental design for a skintreatmentdevice, as shown and described.
FIG. 1 is a perspective view of a skin treatment device showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a left-side elevation view thereof;
FIG. 6 is a right-side elevation view thereof;
FIG. 7 is a bottom plan view thereof;
FIG. 8 is a top plan view thereof showing the skin treatment device with environmental subject matter; and,
FIG. 9 is a bottom perspective view thereof.
The broken lines in shape of small circles within the squares shown in FIG. 9 depict environmental subject matter and form no part of the claimed design. The rest of the broken lines in the drawings depict portions of the skin treatment device that form no part of the claimed design.