Wacky Placks came out in 1974 and were available wrapped in cellophane at  J. C. Penney's, Woolworth's, and several toy stores.

They are twice the size of regular stickers (3.5"x 5"), and a much better example of the original artwork.

The Wacky Placks were assembled in three layers.  The backing is a wood composite covered in glossy paper, and the image itself is on very thick cardboard.  Two cardboard strips are sandwiched between them for a visual 3-D effect.