To clarify further, each of the channels of the material (Diffuse, Opacity, etc.) is a regular image file (JPG, PNG, etc) that you can load by double clicking on the corresponding icon. See Gerry's screenshot.So what you need to create in Photoshop are regular image files. The .iMtl file is created when saved as material in iClone, but it is not essential unless you want to reuse the material for an other prop.