The photos on glass that I received have a type of foamboard mounted on the back. The exposed surface of the board is quite tough. I can think of several ways of attaching a wire, including a pair of stick-on hooks with the wire run between them. A pair of very carefully drilled holes in the backer surface, just big enough to insert a bent wire might work also. On some wooden frames I have run a wire between the tabs that hold the backer in place. It doesn't take much. I don't know how metal photos come finished but I'm sure there should be some way to attach a wire.

Just received the 2 prints that I had fractureme.com print on glass. I placed the order last Mon or Tue and just picked them up at the post office. Not bad, considering it was the week of Thanksgiving. They measure 4.8x6.4, borderless, with a quality backing and a special screw to make it easy to hang. Color is excellent. Printed exactly as I sent it to them. Very well packaged. They can also do them with a desk mount instead of the wall hanging. Everything appears to be high quality. $18 each plus shipping. Their largest size is about 21x28 for $125. All rectangles printed in 4:3 ratio, and they also print squares. I'm happy and will be using them again.