KTX is a texture storage format that is easy to load into GPU.
Effect Constructor includes utilities to convert material textures from PNG into compressed KTX. We use ETC2 hardware texture compression which is best suited for mobile GPUs. This compression is lossy, so it’s better to keep source PNG files for any future modification. If you need to view or edit KTX, on Mac you can use PVRTexTool from PowerVR.
Download PowerVR SDK and choose PVRTexTool during install. It can open all kinds of KTX files, export into PNG for editing in Gimp or any other software, and compress from PNG back into KTX (make sure to use ETC2 compression).