First you may need to prepare the picture... This is not really required of the pic in itself, but to convince you that the procedure here is a safe one (for your phone).
You will need one of those saran wrap rolls for your kitchen.
Get it, and wrap your phone in it. Then wet your hand and sprinkle water on it. Make sure it remains dry.
Then, repeat with some more water, then actually dropping it into a bowl.
If your phone ever gets wet (doing this dare or not), take it apart (phone, battery, and cover) and cover all 3 in a bowl of raw rice, and let it say like that for 2-3 hours. The rice will dry itself properly.
Once you know the wrap will keep it safe, you can do this dare without fear.
Cover it with the saran wrap, activate the phone cam's voice shooting function, set it on the floor (the cam pointing upwards), stand on top of it and open the shower.
Shower as normally, having your phone get plenty of pics...
Distribute them properly.