Use 3L Scrapbook Adhesive Black Photo Corners to quickly, easily and inexpensively display photos without damaging or altering them. They are self-adhesive, conservation-quality, PVC free, and archival safe. A package contains 250 photo corners, which measure 3/8" each.
These corners are a little hard for me to work with because of arthritis in my hands, but after half an hour, I got the hang of it. The adhesive is very strong. I only wish the corners were a little bit larger.