A Legendary Album

Here's a one line script you can paste in chat to check which ones are currently active based on which world quests are up right now:
/run print("Currently Available:")local n={"Abu'Gar","Nat Pagle","Oculeth","Time-Warped Fisher","Chen Stormstout","Elder Clearwater","Wrathion"}for i,v in pairs({0,4,25,35,557,584,624})do if C_TaskQuest.IsActive(70075+v)then print("-",n)end end

It will only print out which photographs you can get during the current world quest rotation.

World quests reset biweekly (NA: Tuesday morning and Friday morning; EU: Wednesday morning and Saturday evening.)
If you are unlucky there may not be any Cataloging world quests active at all.

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• Each world quest will always spawn its specific legendary person in the same spot every time.
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• Make sure you don't complete the world quest until you have your photograph snapped. Then you can retry if you miss it!
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• You do not need to finish the world quest to get credit for the achievement.
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• You can make it easier for yourself by creating a targeting macro and spamming it during the ride
  Spoiler

  
  Here's a macro for Oculeth that you can spam during the ride that will make a sound and a raid warning when you are near him:
  /cleartarget
/tar Chief Telemancer Oculeth
/stopmacro [noexists]
/run PlaySound(8959)RaidNotice_AddMessage(RaidBossEmoteFrame,UnitName("target").." nearby, get your camera ready!",ChatTypeInfo["RAID_WARNING"])
  Change the name after /tar in the second line of the macro to whichever NPC you are currently looking for. Exact names are listed in bold below.
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Where to go for each photo:

Spoiler

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• Chen Stormstout – Waking Shores (east of Obsidian Bulwark)
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• Abu'Gar – Waking Shores (Life Vault Ruins)
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• Wrathion – Thaldrazsus (balloon ride, Algeth'era)
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• Time-Warped Mysterious Fisher – Thaldrazsus (raft, south of Valdrakken)
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• Nat Pagle – Ohn'ahran Plains (The Mallakh)
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• Elder Clearwater – Ohn'ahran Plains (Forkriver Crossing)
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• Chief Telemancer Oculeth – The Azure Span
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Do not complete the quest until you have taken the legendary shot you are after. If the legendary character is not there on the first run, just try another go until they appaer. But do not finish the quest (so either do not take photos or be in a raid group).

4 weeks and no nat pagle or time-warped fisher. the meta achievement strikes again.