Can't help much, but next time use rubber cement. It holds the pics but rubs right off when you take them off. Used it on posters for MANY years.
Just a guess. Try using a Big Pink (pencil Eraser. Don't go cheap! Get the good kind, because some of them today are made with oils that will just make a bigger mess. Should work.
The backs of photos are generally a very light weight plastic. You may want to try a cotton ball or soft cloth dampened with a little Turpentine, WD-40 or Mineral Spirits. Not knowing what kind of glue you used Proceed with gentle steps and experiment with a photo that is replaceable. (do you still have the negatives?)
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