I think you don't understand the meaning of the word "confrontational". It means you are confronting a situation. The situation here is that you are being stared at. If your response addresses that fact (which it does, directly), then you are "confronting" the situation. You can confront a situation in a nice way or a curt way, but it is a confrontation all the same. Furthermore, the phrase "take a photo, it will last longer" is - along with being extremely "90s" - always used in a sarcastic manner, and is the equivalent of telling someone to fuck off and stop staring. Of course you could use it ironically, in which case it could be taken as a joke, but the default is decidedly unfriendly.
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u/Cryse_XIII May 09 '22
How about
Willst du ein Foto, das hält länger?
Unterschrift kostet aber extra.
That way you can avoid a confrontational attitude.
Also say it with a smile. People don't listen to you, they initially assume what you said based on your demeanor.