You'll need to save your custom skin as a DDS file, as there's no auto-conversion like there is for car and helmet skins. You may need to track down a DDS plugin for your painting software, as it's not a standard file format for most people outside the game development world...
Then you need to rename your file so it has the same name as one of the existing skin files, and then replace/overwrite the default file in the data folder with your new skin.
You'll need to distribute that same DDS to anyone else that you want to be able to see your suit/gloves - team-members, etc.
Don't forget to backup the original LFS textures before you go overwriting them, just in case you need to go back