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.