The OP doesn't specify an age, but we'll go with teens. They know when they're up to something they shouldn't be. That's why they excel at sneaking. If they post porn and get caught, they will have to answer.
Keep in mind also my other point. Such teens may be making themselves targets of child pornographers, to be expoited. I'm sorry, but I cannot agree this is pretty normal behavior.
Anyone in possession of the photo, wary or not is also at legal risk. Suppose your teen daughter is using your phone, and a teenage boyfriend of hers sends her a pic of himself. That pic is now on your phone and YOU are now in possession of child porn. Good luck.
I'm not an attorney but juvie records can be sealed. It's not necessarily a lifetime tag, but a kid who does this is risking legal trouble. Any judge will order counselling for sexual behavior.