Thanks everyone, making sense now. When I first google translated "Gepruft" it translated into "checked". I initially thought the photo (on backside) was cut out of a postcard hence the deckled edge. Now I can understand that these photos were sent to my Dad (Polish POW) in the Stalags, probably in a letter and the contents were checked and stamped.
Now I see that over his 6 years as POW, the pics with stamps from 1939-40 were well worn (Stalag XXA) and the later ones were more readable (Stalag VI J). To conclude, I added another stamp from Stalag VI J with a Swan icon. Thanks again Roman Frackiewicz