Submitting pictures you didn't photograph

How do members feel generally about the submission of Old Postcards, Old Photos, Paintings etc., to Pictures of England, which the submitter hasn't actually photographed themselves? 

Is this cheating? Or does it add to the archival content?

Would it better to leave them off the site and only accept photographs which the submitter can guarantee they have taken themselves?

My thoughts are that provided you state that it is a postcard printed/issued by whoever etc and making it clear that it is NOT your own work, then you should be okay.

Of course when you become a member of POE you warrant that the picture submitted is yours, so maybe a grey area, and one to be given further thought maybe?

I have submitted a friend and a cousin's photo's. I always ask their permission, and then credit them with the picture.