I cut and pasted images from a magazine (Lodown - graphics/music mag) onto the coasters using extra strong glue.

Smoothed all the edges down with a nail file so they 'blended' into the cork with no rough edges and then coated them with about 4 coats of matt varnish.

I alternated the brush strokes on each coat of varnish to give a subtle effect - can't see it very well on the photos though!