You need to have completed the main quest “Divine Beast Vah Ruta” before this sidequest will open up. Find out about how to calm Vah Ruta here. You will also need to have completed the main quest “Locked Mementos” so you can have access to your Sheikah Slate’s camera, which you can find out about here.
Talk to Leflat who will talk about Zoras being reckless and going near Ploymous Mountain. To try and deter them, she wants you to take a photo of a Lynel (not any of the subspecies, just a regular Lynel). The photo has to be in your photo album. If you took a photo of a Lynel in the past for your Hyrule Compedium but deleted the original photo, you will have to go take another one.
Leflat will mention a Lynel living in Ploymous Mountain but you do not have to take a photo of that specific Lynel. If you just so happen to have an undeleted Lynel photo on your Sheikah Slate, then talk to her again to complete the sidequest. If not, then Ploymous Mountain is the closest place to the Zora’s Domain and the easiest to access using the Zora Armor.
Once you have the photo, speak to her again and she will give you the Zora Greaves, one of the pieces of the Zora armor set.
Note: If you want to complete the Zora armor set, at this point you are likely missing the Zora Helm. To get the Zora Helm, travel to Toto Lake and go out to where there are ruins in the water. Around the center of the lake, the chest is completely submerged. Pull it up with Magnesis and open it for the Zora Helm!