<<Did the Natzis ever make use of FDR's health problems?>>
Did they know? They would have had a credibility problem anyway because before the U.S. got into the war, the Nazis bent over backwards to avoid offending them. If they suddenly "discovered" after hostilities broke out with America that FDR was mostly wheelchair-bound, it would be hard to explain how or why this fact had never been previously disclosed. Might also revive the rumors that, as the song says, "Hitler . . . has only got one ball." Or that Goebbels was hobbling around on a club-foot. Not what you'd come to expect from Aryan Supermen.
<<I know that FDR was carefull to controll his image and photos of him sitting in a wheelchair are scarce , this might be why. >>
FDR was anxious to portray an image of strength and vigor to the domestic audience. And I understand that the press was fully cooperative and knew not to publish wheelchair photos of the Prez. It was not entirely an issue of FDR "controlling" them. They did not WANT to embarrass him, at least not in that way. I think there was a stronger code of ethics then for journalism, at least with regard to protecting the personal lives of public figures regardless of politics.