Pictures are not enough. Adultery isn't the dating side of cheating - it's the sex, and your pictures don't prove that. There is very little acceptable evidence - a child of his born to another woman is one example.
I divorced my ex for Adultery - with prostitutes. I didn't have to name anyone or be very specific. In the box on the form I just wrote that he had had sex with prostitutes on various dates in the previous 6 months.
He could have argued, but he didn't.
If I were you, I would tell him that you know he is cheating, and tell him about the PI if he denies it.
Don't mention anything about evidence for the divorce. Then see a solicitor to get the petition drawn up and worded.
You might actually find that once you serve the papers, he doesn't argue it.
Don't name the woman - it means paperwork goes to her too, which slows everything down. It might be a card up your sleeve though, if he doesn't want her name on record - you could intimate that if he fights it, you'll name her.
But keep in my you can get rid of the fucker just as quickly with Unreasonsble Behaviour. And for that, he's allowed to say he doesn't agree but it still proceeds.
So you can put in on record as an example "Mr Arsehole regularly spent time with another woman - dinners, hotels instead of spending time with his wife, this led to the assumption of adultery".
A solicitor will help with the wording, but even if he denies it, you can get it in there 
Good luck getting rid of this scum!
And remember that getting rid is the important bit. Admitting adultery to the court doesn't change the divorce at all. Remember that you know what he is - you don't need him to admit it or any court to stamp the paper that says it. Keep your head high.