In Gen 1:26, God says man is made "in our image" but after the fall God says the man has "become like ONE of us in knowing good and evil" What does He mean?

Wow, this is going to be hard to keep short. I'll try. Soooooo much to talk about here. 

The plurality in 1:26 refers to the Godhead. God is 1, but exists as 3 Persons: Father, Son, Holy Spirit. We are 1, but 3 as well, you could say, as we are body, mind, and spirit. There is more to you than what you see when you look in the mirror. It's a hard analogy, as is anything trying to compare to the Almighty God.

The statement after the Fall, in my understanding, is just pointing to the Godhead, so to rephrase it, “man knows the difference between good and evil as does God.”

BUT, I heard one preacher put it this way: God knows evil like sunbeam on sewage – untouched, unfouled. Man knows evil like someone drowning in sewage.

That's all the further I take 3:22 – I don't read anything more into it, personally.