So what exactly did we gain when we started using the gym_leader and elite_four tag in the first place? I can't find them ever being discussed here. The characters don't have anything (visibly) in common either.
I also like the tag for when it shows a single gym leader and their pokemon, such as post #583618 and post #900516. Posts with much or all of their roster, an authoritative pose, or maybe set in the middle of or just before battle.
"Posts with much or all of their roster, an authoritative pose, or maybe set in the middle of or just before battle."
This sounds very subjective to me. A pool would be better for this description.
Here's my simple analysis of two images of Kasumi:
post #942945 would deserve to be there. She has only one Pokémon (Starmie) in that image, but she looks ready to fight, that Pokémon is a recurring member of her teams as a Gym Leader in the games, and she has two Cascade Badges and two Pokéballs.
post #573718 would not be there. All the six Pokémon she has at some point in the anime appear in the image, but in the anime Misty almost never takes the role of a Gym Leader.
Log, I should have been more specific. I'll try to clarify. I didn't mean that the specific criteria ("much or all of their roster", "authoritative pose", etc.) are subjective. They are subjective to some extent, sure, but I was not criticizing this exactly.
The task of deciding what should be the criteria for a good collection of images of gym leaders is subjective, therefore I believe a pool would be better to fulfill it.
There is the problem of Gym Leaders who take other major roles, such as Ookido Green, but we don't need to focus on that now. It is usually clear enough what is the role of the character in an image. Instead, I'll focus on the criteria people wrote here.
In my mind, Bloodletter's criteria ("Posts with much or all of their roster, an authoritative pose, or maybe set in the middle of or just before battle.") are a very good start.
Log's reply presents criteria that are a little different from the previous ones ("Does this image have four or more gym leaders?", "Does this image have only one gym leader but all or all minus one of their pokemon?") and these are, yes, very very objective, but not perfect; they could be improved.
post #724509 fails Log's criteria, because Sakaki is with only one Pokémon, yet to me it really looks he is taking the role of a gym leader. Probably it doesn't fail Bloodletter's criteria; I'd like to know if they agree with me.