You should definitely dump him. Quickly. And don't think twice about it.
You're jolly ole fellow is probably already cheating on you. Plus, he's not that smart. I hate to say this, but if some chick were to send me some inappropriate picture (which isn't happening), the first thing I'd do is erase it. In today's day and age of website hackings and people finding new and creative ways to snoop through their companion's lives, email isn't safe. So he's just not that bright for one. And you don't want to date dumb chaps.
Also, the fact that he lied about it indicates he's up on the greatest shut-down-a-convo tool of all time: deny, deny, deny. However, you have proof. He can't even lie about it. The fact that he'd be so cavalier as to lie to your face means he doesn't respect you (well that and trading nudey pics with the coworker). I wouldn't be surprised if its just a prelude to some in-person fondlage that they have planned for a later date. All cheating starts innocently enough.
Those two? Guilty as the day is long.
Drop him before he hands you an STD.
DUMB HIM ASAP
lmao i had a similar situation happen plus my ex boyfriend is as dumb as this retard they're talking about maybe my ex and this dude should hang out
DUMP HIM!
And get a new, hotter boyfriend.
I agree with the answer however, I noticed not a single person has mentioned the breach of trust of her logging into his personal account! That's just as wrong. It seems to me, neither of them have much respect for one another and it would probably be best for a parting of the ways.