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I'm not sure you could use decoupage glue over a photo , but here is an interesting idea for attaching photos to the bottom of wine glasses using decoupage glue on the edges:

http://www.kodak.com/eknec/PageQuerier.jhtml?pq-pa...

You can use photo transfer paper instead of the photo itself:

http://www.ehow.com/how_4792471_photo-transfer-pap...

Or use inkjet shrink sheets:

http://www.ehow.com/how_5683203_put-photo-glass-ca...

I also found this ultra violet light way of adhering photos to glass:

http://www.glass-etching-kits.com/new_glass_pictur...

HTH

Shot in the dark here, but, I wonder if wall paper glue would work. I'm thinking that it has to be something that dries invisible, and off the top of my head that is the only thing that I can think of.

Nothing. If you glue the face of the photo you will ruin it. Can you use sticky tape discreetly on the edges?

Glue For Photo Paper