if you make an image to show your mat pose...just render one out, at a square ratio,....then take the render to your image editor, photoshop, photopaint, whatever, re-size it to 91x91 pixels, and apply an active mask over the whole image. then save as type .png, with the same name exactly as the corresponding .pz2 file. This allows you to place a good image into a thumbnail for the libraries. hope this helps