Very nice photo location. You just need to follow the gravel road from the parking lot, it takes just a few minutes to get to the falls. It's work going all the way above Reykjafoss, so you can also see waterfall with the canyon in front of it. And just a few steps from there you will also find Fosslaug hot spring.
Paid Parking/entrance 1.000 isk (honesty box/person with a payment processor)
Toilets next to the parking lot