@PotomacBobWhilst not specifically the situation the OP stated, and there is a caveat that legal laws are different in every country, there has been multiple court cases over the years in the UK about various people above and below legal age receiving unsolicited images of both nudity and violence.
Their defence has inevitably been that they did not ask to be sent or seek out those images and deleted them straight away. Which on the surface, appears fair, however, not in the eyes of the legal system. If you receive images that are (or you believe to be) of an illegal nature, then deleting them is not (in the eyes of the justice system) an acceptable course of action. The LEGAL course of action is to report the images/content to the correct authority as soon as possible. Deleting the aforementioned images will both make you an accessory to destroying evidence as well as being party to failing to report a crime.
The severity of the punishment is determined by your age and profession. For instance, a sexually explicit picture of an underage girl was circulated amongst a whatsapp group made up of Met police officers a few years ago.
One officer reported the image and all those that received the image (but didn't report it) were instantly sacked with several-if memory serves right- receiving custodial sentences for the possession of CP (even though it was one image). Even though several used the "I didn't ask for it and it was unsolicited, and I deleted it straight away.." defence. It didn't work.
The courts in the UK take a dim view of that as an excuse.
Obviously, given their profession, the police were given absolutely no lee-way, nor did arguments by their lawyers that given the nature of their jobs, they would be targets within the prison system, sway the sentencing judge.
Ignorance, as they say, is not a defence...
Oh, and in relation to the Met example (one of many-they just don't seem able to learn...) Whilst several hadn't opened the attachment and were able to prove that, they were still found guilty as they hadn't reported the message in the first place and couldn't conclusively prove to the court that they didn't KNOW what the content was, and that the act of deleting the attachment without opening it, implied a prior knowledge as to the content. Which brought them full circle back round to being in possession of illegal content and failing to act on that as per their job...