Minecraft wiki entry for Custom_texture_packs, associated text: The textures are in the Portable Network Graphics (PNG) format and can be edited with any image editing program that can handle the format including transparency. Many different programs support this, some free ones include Paint.net (Windows) (which even Notch uses), GIMP (Windows, Mac, Linux), Paintbrush (Mac), and many more. Edit each PNG file as desired and save them with 32-bit color depth to preserve transparency.