But a good learning experience since I am betting you already know!
The first picture looks like a more mature 2-3 year old as compared to the second picture. If correct, I'd "guess" that pic 1 is a male and pic 2 a female (but not as matured). Of course, while I had them "flipped", I'dd "attempt" to "milk" both. The problem is that at this time of year, you may not get anything out and in the case of the second pic, if my "guess" is correct, not getting anything may not mean anything either since the Koi's vent indicates that it may not have reached sexual maturity yet.
All just guesses, based on the pictures and I am typically wrong when guessing from pictures!
Steve