If someone admits in a message to me they are using someone else's pictures (not their own)...who do I report that to?

The same way you would report a scammer. You would see the option to report their pic once you click on Report.

But...do they *read* what you write..?
Like....they need to read where he says to me "oh, those pics aren't of me.."

I report fake pics that way. Just give Admin all the info you can in the ‘Additional Comments’ box. I normally do a reverse image on the pic and then include the link where the pic can be found on the web.

I usually do that too, when I am suspect about the pictures..
But this person ~told me~ they are not of him...yet, when I did a reverse search...nothing came up...

I’d still report them stating that particular member admits to using fake pics.

I'm curious why you would dedicate so much time to someone who admits they lied to you?
Just report them and put them in the past.
Why waste energy on them?

I also don't see the benefit of any one person reporting anything in detail.
As if Mingle2 actually cares? If they did, it would show.

Use the tool and block the profiles which do not apply to you and give in no further energy. Focusing your time and energy to something which does not advance your goal makes no sense to me?