You need to create a PNG image file from the substance and save it in the same folder - using the same name as the substance file. These images will then show inside iClone.

You can create the image up to 512 x 512 pixels, and it works quite well if the image is 125 x 125 pixels as this saves space on your hard drive. The images can be generated in Substance Designer.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.