Juannelle in the Snowfield Stable wants to see what a Stalhorse looks like. She has heard that the Stalhorse appears in the N. Tabantha Snowfield, as well as to the east near some bizarre ruins. Link can find a Stalhorse near the skulls, located in the snowfield north of the Snowfield Stable. Alternatively, Link can find a Stalhorse just outside of the North Lomei Labyrinth well to the east. Much like the Stalkoblins that ride the horses, these can only be found at nighttime and will disappear when the sun rises.
Make your way to either location and take a picture of one of the Stalhorses. Then return to the Snowfield Stable and show the picture to Juannelle. As a reward for helping her find a Stalhorse, she will give Link a Silver Rupee.
Speak with Juannelle to start the quest.
When you showed the picture of the Stalhorse to Juannelle, she carefully traced it so she'd have her own copy.
It seems she plans on using the picture in an article she's writing. With the picture for inspiration, she's excited to get to work!