Speak with Juannelle at the Snowfield Stable. She wants a photo of a Stalhorse, a skeletal horse that only appears at night and turns to dust in the morning.
Juannelle will tell you that Stalhorses are sometimes sighted in the North Tabantha Snowfield. In order to succeed in this sidequest, you will need to make sure the image is in your actual Album, not just the Hyrule Compedium. And make sure you hunt at night, since obviously Stalhorses die in the morning.
Between 9:00 PM and 5:00 AM, go north of the stable towards North Tabantha Snowfield and you will see an enemy camp marked by a large skull-shaped rock. Take a picture of the Stalhorses that the Bokoblins are riding on. You do not need to knock the Bokoblins off the Stalhorse. Then you can return to the stable and show the photo to Juannelle.
You can even ride one back if you want. Just bear in mind that Stalhorses cannot be registered and will die by 5:00 AM.
Once Juannelle has gotten enough of the photo, she will give you a silver Rupee worth 100 Rupees as thanks.