for essentials, there are a few key things you need to know.
1: Sprites in essentials can be any size you want them to be, in any style you prefer. The game can Draw any sizes prite, but the upper limit is the game windows max size without things being cut off
2: for each pokemon in the pokedex, there will be 5 unique sprites, only 1 of which is required for the game to not break, that being the Icon sprite located in "[Game Folder name]/Graphics/Icon.
3: for each entry in the pokedex, you should have 4 battle sprites at bare minimum
4: to match the pokemon's sprite to it's pokemon data, it's National dex value must match the number assigned to the sprite. (Ex. Mew is National value 151, so all sprites that depict Mew should be called "151.png")
5: the naming system in base essentials is weird. There are 5 'attributes' a pokemon will have to define which sprite should be called upon, and only 2 of them need additional data elsewhere in the game code/pbs [essentials version dependant] these attributes are as followed: Back, Shiny, Female, Form, and Shadow.
6: For each of the 5 attributes the sprite needs to be named in a specific order to show up in game. improper naming [B]will result[\B] in that specific sprite not working properly. here are examples of the orders with some variation to differentiate between them (XXX refers to the pokemon's national dex number):
XXX.png = The Pokemons normal front sprite [Opponent, summary, and box sprite]
XXXs.png = The Pokemons Front Shiny Sprite
XXXf.png = The Pokemons Female variant
XXXb.png = The Pokemons Back sprite [only seen in battles for your or an ally's pokemon]
XXXfb.png = The pokemons Female Back sprite
XXXsb.png = The Pokemons Shiny Back sprite
XXXfsb.png = The Pokemons Female Shiny Back sprite
XXX_#.png = The Pokemons front sprite in an alternate form [requires modification of pokemonforms.txt/Multiform script section (Essentials dependant) to be used] [# refers to the pokemon's form number. this has no known limit]
XXX_shadow.png = The Pokemons Shadow Front Sprite
XXXfsb_#_shadow = The pokemons Female Shiny Back Shadow Form sprite.
The absence of a feature will result in the sprite being drawn in the nearest available sprite.
(Ex: A Shiny Female Shadow Frillish will result in the normal Shiny Female Frillish sprite being shown in battle)
There is also an "Egg" Sprite that can be made. all it requires is XXX_Egg.png to function
Note: the above rules I mentioned can also be applied to the pokemon's Icon Sprite that displays in the Party and in the Storage system. all you have to do is apply the prefix "Icon" before the national dex number:
IconXXX.png [include the other attributes too if you wanna do that]
I hope this in depth guide to sprite used in essentials has been helpful