What the best way to attach pictures to an old window?

You could try ModPodge

Mod Podge is generally used.

If you don't want the photos to be damaged, you can put them in old frames and glue the frames to the window glass. That way you can change the photos, protect them and have frames within frames.

https://www.wikihow.com/Create-a-Hanging-Picture-Frame-from-an-Old-Window

You could treat it like it is a picture frame and mount glass on the backside that is removeable and put the pictures between the glass. This way you could switch out the pictures with updated pictures, or choose themes and switch them out by seasons, sports, family trips, etc.

Depending on how important the picture is would determine what I would use. If a Polaroid or a copy, make an inkjet copy ...use a copy store such as Office Max...otherwise the ink on the original will smear. I vote for frames where you can change the pictures. I am a professional decoupage and while I love doing this, unless you want to devote much time to practice, I would not recommend this as a first project.

Did you try spray photo mounting glue? It not only holds really well, but gets the wrinkles out, too and is totally bubble-free

I did something similar for my folks. I enlarged the images to fill the spaces and put the photos behind the glass. I just used packing tape to secure them.

http://www.hometalk.com/diy/decorate/walls/how-to-turn-an-old-door-into-a-photo-gallery-9635783

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