The ornamental design for a lampshade accessory, as shown and described.
FIG. 1 is a perspective view of a lamp shade accessory showing my new design attached to a base of an existing lamp shade;
FIG. 2 is top plan view thereof;
FIG. 3 is a side elevational view thereof;
FIG. 4 is a perspective view of the present invention adapted to an annular shape; and,
FIG. 5 is a perspective view of the present invention adapted to a linear shape.
The indicia or broken lines shown on the figures are for illustrative purposes only and are hereby disclaimed as forming no part of Applicant's claimed invention.