Hi, and thanks for your question.
3M Super 77 Spray Adhesive forms a permanent bond once it dries, so I'm afraid you won't be able to successfully remove the photo from the foam core without ruining it. If you have already tried peeling the photo slowly, without damaging it, you can continue to try that, as long as you proceed very slowly, and the photo doesn't tear.
However, in order to preserve the photo, you might do better in cutting out (around) the piece of foam core on which the photo is mounted, and keeping it that way, or putting it in a frame, if that was your intention. If the photo is irreplaceable and you would like to keep it without the foam core 'backing', you could try scanning it into your computer (once it's cut and still on the foam core material) and printing it as a photo on photo paper, or if your computer/printer doesn't have this capability, you can bring it to a printing place like Kinkos, that can do it for you. You could also try 'thinning out' the foam core behind the photo with a Kraft knife or Exacto knife, VERY carefully, once it's cut out.
I wish you much good luck in preserving this photo!
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