lines from firends(sitcom)
Monica: I said that you had a nice butt, it's just not a great butt.
Joey: Oh, you wouldn't know a great butt if it came up and bit ya.
Ross: There's an image.

Q1: "if it came up and bit ya"?
What is "it"
Does "if" mean that "even if" here?

Q2: There's an image.
After he said "There's an image.", the audience laughed loudly. why? and what does "image" mean here?

Question

lines from firends(sitcom)
Monica: I said that you had a nice butt, it's just not a great butt.
Joey: Oh, you wouldn't know a great butt if it came up and bit ya.
Ross: There's an image.

Q1:   "if it came up and bit ya"? 
What is "it"
Does "if" mean that "even if" here?

Q2: There's an image.
After he said "There's an image.", the audience laughed loudly. why? and what does "image" mean here?

Answer

Question 1:
"It" refers to "a great butt"
He means that she wouldn't recognize "a great butt"
People use this to say you wouldn't recognize something.

Question 2:
Ross was picturing her getting bit by "a great butt" which isn't appealing or that is absurd.
He is sarcastically proclaiming that, the image is something that you don't want to see.
People say that sometimes .

Answer

Q1: "if it came up and bit ya"? 
What is "it" = "it" refers to the "great butt" that they are talking about.
Does "if" mean that "even if" here? = Yes

Q2: There's an image.
After he said "There's an image.", the audience laughed loudly. why? and what does "image" mean here?

"Image," here, means the picture of a butt biting someone on their butt. Why? The audience laughs because the mental image of someone's butt biting someone else's butt is, supposedly, funny.

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