criminal offence, 10 minutes' drive from a memorial and cemetry where several dozen Nazi-executed ancestors of people I meet on a daily basis are buried.
You are correct that displaying these images in ignorance does not by default make one a racist. But displaying them with full knowledge of their historical significance and then CLAIMING THIS IS FUNNY? Imho anyone taking this line is in significant ways more despicable and more dangerous than an overt political racist.
To trivialise recent history of this gravity is to pass a line beyond which I believe society has full right to pass judgment upon you. I can but be thankful that the overwhelming concensus judgment of the interested community is that this behaviour is odious; you should be thankful that your association with people like this has been in an online space where you have been able to avoid the very real repercussions that acting like this in the real world would bring.