Be careful with those words.
Firstly, the boss may have been sent these as attachments for a purpose and not realized they were not destroyed.
Secondly, there is a fine line between porn (of any age) and art. Were this the computer for the curator of a museam, the pictures could have been sent as samples of an artist's work etc..
while very unlikely this is the case ... always give advice on the side of caution.
use of the conjunction "IF" is called for... that is "IF this is criminal OR deviant"
The person should know IF porn or potentially pornographic content is commonly allowed on company computers. That would be in the employee handbook.
If porn (or potentially pornographic) material is not allowed as a company policy, then the employee should notify his direct boss so long as that is not the associated porn holder. With that, his boss would be required to handle it as part of an HR issue, HR would then be responsible to calling in law enforcement or classifying how the company will deal (if HR needs to they would consult the company lawyers)
Given all of that, if the matter were not resolved satisfactorally to the employee, ie, they tell you they've taken administrative action against the CEO or are cleaning it up and so forth ... then the position is how you want to deal with it.. you can contact the police, they can come and talk to the people you reported it to inside the company.
This is the way to handle the issue. Don't jump to conclusions.
I was investigating an office for a company I work for, "slow internet access" was the complaint. During *my* investigation I found hits to a porn house, the company policy is clear that non-business use of company equipement is forbidden (and the company is not in porn or graphic media or any remotely associated porn industry/side bar)
With that as part of my finding for the "slow internet" and that I could identify the computer, I verified the currently logged on employee, looked up track records on this computer where there had been issues in the past that it was "cleaned" of spy and ad ware (conclusion now was that it was never checked for porn before and that the employee was visiting porn sights which downloaded the spyware on him) and with that information, fact and conjecture, I went to my boss.
He went to his boss (instead of HR as he should have) and his boss said get additional information/log activity, scan drive directly, etc..
with orders in hand I covertly scanned, obtained proof of access and content. With that handed off the packet to my boss who then handed off to his, and the employee (one of several company owners) was taken to a legal counselling session. We (I.T. Dept) were told in future to handle these issues the same way, with exceptions on who we would notify if it was our direct superior etc... because I was directly involved the company kept me informed of their decision.
Granted had it been child porn and I not been kept in the loop I would have informed the company they needed to, and then told them that if the outcome was not given to the police for formal investigation, I would have no alternative myself (since in collecting the information I would then have copies of it too and could be implicated) It is also for that reason I keep copies of my email off company site so that I can have unaltered copies to help keep me clear.. etc..
These are better and more complete ways on handling such issues.. if the guy has porn that violates company policy, its however the company wants to deal with it. if the porn clearly broke law (child or otherwise) then I would have expected the outcome to have involved the police. Even if the guy is innocent and it were planted.. because at that point, the law would go after the purpetrator.