Start this star by jumping in at the very bottom of the painting, so the water level is at its lowest. You will be heading to that one cage, which has a breakable box at the bottom of it. In fact, the first thing you should do is head over there and break that block. This reveals a hole that allows you to enter the cage, but there’s nothing there. Instead, you will need to reach the top of the cage, but raising the water level will make things a lot more complicated, so it’s best to avoid that. In fact, it’s possible to reach the ramp right above the bottom of the cage.
It will take a Triple Jump and you might need a few tries, but it’s definitely possible. The best angle to approach the Triple Jump from is by starting just to the right of the opening at the base of the cage. Once you get onto the ramp, head to the top of it and you’ll be near the purple switch up there. Hit the switch, then use the blocks to get on top of the cage. While you’re up there, you should notice two wooden planks sticking out of the side of the cage, which are connected to pieces inside. One of the pieces inside has the star on it, but it doesn’t move.
However, the other platform will move whenever you step on it. In fact, it will move all the way to the bottom of the cage. So, what you need to do is step on it to get the “elevator” moving, then quickly drop down to the ground. Enter the cage and jump on the wooden platform, either via a Side Somersault or Wall Jump. You only have a limited time to do this, by the way, as the wooden platform will return to the top. If you’re on it when it does return to the top, then you will be able to grab the star.