People seem slightly confused by the legal side of this.
The girl has put these photos on facebook.
Not only that, but she's made them her profile picture.
Your profile picture on facebook is public domain.
There is no way in hell anyone would ever win a court case made up of "He saved my profile picture and I don't like that he saved it".
She put the pictures up. It's entirely possible that she hasn't even set her privacy settings to maximum, so complete strangers may also be able to see her timeline pictures. If so, her timeline pictures are also public domain. Nobody can be prosecuted for saving the pictures that someone publishes in the public domain.
Now if your DH was using those pictures to coerce, intimidate, threaten or blackmail this girl, that would be legally actionable. As it stands, keeping them on a flash drive is not a crime. Nor is it proof of intention to commit a crime.
This is not illegal.
It might be immoral, it might be unsavoury, it might be distasteful, but it is not illegal.
That said...
If your husband says he never wanks, then I'd say it's 99% likely that he's lying. It is far more believable to me (as a man) that he is simply uncomfortable talking about it with you, and lies in order to avoid being uncomfortable.
Ditto any conversation about porn. I'm far more likely to believe that someone denies ever looking at porn because they're worried about the impact on their relationship, rather than because they genuinely never look at it.
Here's the important bit:
None of the above means that what he is doing is okay
It's worrying that he's keeping pictures of a colleague on a flash drive.
It's worrying that he's having more-than-usual contact with her, in his office or otherwise.
It's worrying that he feels the need to lie to you about wanking, porn, or anything else.
It's worrying that you seem unable to have a discussion together without him blaming you for everything. This isn't normal behaviour.
It's worrying that he's carrying this around on a flash drive. This would seem to indicate he's intending to access these images on a device other than his own computer. If he was just using it on his own computer it would make far more sense to just have a buried folder somewhere on C:
What's your sex life like outside of this? Do you have sex regularly? Are you both happy with the frequency? Are you both happy with the methodology? It might be worth tackling that as a first step, and then the porn/pictures may well go away of their own accord. I understand that 4 kids doesn't inspire a huge amount of lust, but two people having a decent sex life shouldn't find situations like this arising, so you could do worse than trying to solve that aspect of the relationship before accusing him of being a creepy pervert. He might be one, but unless you're happy to divorce him on the spot over it then bringing it up in an argument doesn't seem to benefit anyone.