Usually this is not recommended. Photo prints vary a lot by paper, coating, and printing process, so there’s no universal guarantee that a finished photo print will survive the heat and path through a laser printer. It could damage the print, or in the worst case cause problems in the printer. If you try it, you’re taking that risk.

Safer alternatives suggested were:

• use labels/stickers that are rated for laser printers
• use another printing method for the address area
• print the entire postcard on suitable laser-printable stock instead of printing on finished photo paper afterward

If these are being mailed, also consider that a traditional photo print may not stay pristine through postal handling anyway. For many postcard uses, printing both sides on card stock with a good color laser printer may be more practical than using separate photo prints.