help needed on saving and putting skin ingame

[ukBullTerrier]

hay, i just need some help on how to put skins in game and a few basic thing.
i know how to paint as ive done it before, ive already made a nice skin for a project im doing, however when i look into the skins folders they look very messy and i cant get my head round afew things.

firstly, when im saving a skin ive made in gimp, what is the file type to save it as?

also once ive made a skin, what do i do next?
as when i looked in the skin folder there seemed to be alot of files with the same name, like they are layered or something? or perhaps placeholders for the skin you use?

any help would be great.