I have been doing a lot of these magnets and this is what I found out through lots of trial and help from another woman at Etsy. I would wrap your picture in packing tape and use epoxy glue instead of the glue they used (I just don't think it is strong enough) in that tutorial to glue the magnet to the glass gem. I would also put a piece of card stock between you pic and your magnet or your magnet will bleed through the pic.
I would not use e6000. Your pic will look good at first but after a day or so when it is dry the e6000 will have eaten through your tape and distorted your picture. If you don't put tape on the pic then the epoxy will distort the pic too.