[Hoborific]

You can replace default skins with your own skins in the CUSTOM.IMG, but there is no way to add your own skin without all players needing the image, if you did distribute the image and a stranger joined the server and seen you using say, skin 303, they'd crash. and this is very player unfriendly.

tl;dr, You can't mod your game and others see it, that's only possible through MTA where the server mandatory downloads the mods.