Clear ~ permanent ~ small diagonal near-invisible photocorners ~ box of 250
We find it easiest to put the two lower corners onto the picture, align it into the desired position, and then stick down. Gently bend the picture forward, place the top two corners and press down.
Corners have the advantage over glue in that pictures can be removed, and there are no harmful impurities in these corners as there usually is in glue. These corners are acid free and near invisible once in place.