I used to make these all the time. The picture needs to be thin (either paper or a can) and cut to fit the glass. You must use silicon glue for this project!
Put a dot of the glue on bottom of the glass and press down on the image. One shot so make sure you line it up correctly. The silicon glue will spread out and any bubbles will disappear when dry. For a necklace, glue a metal eye hook and then a cut piece of felt on the back with E6000.