To install it you must place the image "Playable_Bullet_Man" in this file : Program Files (x86)/Steam/steamapps/common/Enter the Gungeon/Ressources/sprites
Personaly I used "mod the gungeon" to create this file and I don't know if there is any other way to do it.

This is my first finished EtG skin so please leave me a comment if you like it or if you think there is something to fix.