It's about time we talk about textures and how they can be handled in Unity. We will start by listing supported formats, discuss useful tips to consider during exporting, and take a close look at options available upon importing.
Unity has a wide range of supported formats for textures; they include PSD, TIFF, JPG, TGA, PNG, GIF, BMP, IFF, and PICT. Yes, multilayered files are supported, and you can easily use them without any fear of memory increase or performance loss. That being said, don't expect to be able to take advantage of a multilayered format, because Unity will convert the files and flatten all layers. This conversion is purely internal and your source files will not be altered in any way, allowing you to continue to iterate on them.