can you get in trouble for possessing underage nudes of yourself?

Not for possessing but for disseminating. So if you don't show anyone else you should be fine. 

Yes.  And also, if you are still under 18.  Having possession of any underage nude of anyone is illegal, regardless of whether you are underage yourself and regardless of whether you are the person in them.

Yes, it is a crime for you to show underage photos even if it is of yourself. 

Yes you can, and it has happened. 

i don’t think so

I don't know. I have one. Some picture of me standing in the living room when I was like 2 or 3 maybe. It's totally a "look at me!" and my mom just so happened to have a camera thing. It's in a photo album along with a ton of other pictures of me with clothes on. Not sure that is specifically illegal or not, but it does make it a little more embarrassing when your wife and her sister and looking through the album, though. But who cares, if you're really worried about the size of your junk you'd have to be a retard to assume A) your sister-in-law gives a crap and/or B) she thinks everything on your body has grown since then but that one specific thing. One picture, don't post it to the Internet or hand out copies and you'll be fine.

yes...