So when I make a texture I want to put on uke's joints for joint textures, what do I save the file as to put in uke's custom folder?

Click on the right button in the toribash shortcut and choose open file location or C:/Games/Toribash-x.x.
Click Custom -> uke and Copy someone textures to uke folder.
(Recommendation: make the textures by yourself, Save it as targa or .TGA). Done