Maybe a bit of a weird question, but with some family deaths I (well, my parents) recently inherited a bunch of weird postcards that just have POW on them from either WW1 or WW2 (western front, we're belgian). Some of them have some identifying information (place/camp) but not enough for us to have an idea whether or not the people on the card were in some way known to our relatives. Not all of them have an address or text on the back either. Is there a specific reason these were made?
Briefly, what you are describing sound a lot prisoner of war (POW) cards from the International Committee of the Red Cross (ICRC). The ICRC played a major role in coordinating information about POWs, as well casualties that were processed by the foreign powers. They now maintain a database of records related to World War I online. Generally there were two types of postcards that were created during WWI.
The first type of cards that were produced were intended to communicate the status of service members to next of kin which would be fairly terse and effectively just let you know someone was a POW. To the best of my knowledge these didn't have any imagery on them, and there might not even be that much text on them / allowed on them.
The second type served a somewhat more informative role and were illustrated with scenes from POW camps and intended to give the public and next-of-kin and idea of the conditions in the camps and the work that the ICRC was doing. An archive of these is now also online and if you look through it you might be able to find some of the postcards that you have.