These bananas are on the northern half of Monkey Island.
How can I get there?
You're going to want to visit the cannibal village.
When you're in the rowboat, go to the north-most screen. (It looks like a duck's head.)
The village is in what looks like the duck's eye.
Some bananas should be near.
Have you walked to the leftmost side of the cannibal's village?
There's a bowl of fruit.
Take the bananas from the bowl of fruit.