If dry mounting is appropriate, Kool Tack's boards and low-temperature adhesives are good choices. Kool Tack Preserve or ArtCare Restore are especially effective when used with the Perma-Lon Release Board.
The heat of dry mounting, and the possibility of chemical instability of the board or adhesive may not be the main issues. If the art/photo is expected to endure as a collectible, then any sort of dry mounting would represent a permanent change of its condition, which could diminish its long-term value. In that case, and if the substrate is in good condition, then a non-invasive, reversible mounting method may be preferred (not to mention less costly); such as hinges or edge supports.