A photograph of a male nude showing all his exterior genitalia explicitly, full frontal. This is often referred to as "the full Monty", but that term's chief reference is to male-stripper performances, while "junk shot" refers specifically to photographs or videos.
"Junk" has been slang for "penis and testicles" for a number of years, and "shot" in this sense refers to photography.
"You know that famous nude magazine for straight women and gay men? Levi Johnston disappointed me. He was the main article, but no junk shot of any kind. The centerfold had him showing his bottom."
"Bummer. Pun very much intended."
scientist: uhhhhhhh ummmmm OH! how bout we throw trash into it we can call it the junk shot!!! *chucks shit in pipe to oil spill hole*
Nate: Can't, I lied about my report card and my parents found out. Now I'm grounded...
Andy: Wow, that plan was a junk shot... I don't feel sorry for you at all...