The Spherize Textures assistant is a program that converts a square image (texture) to a circular image with a shadow gradient. The image is additionally altered in such a way that it resembles a spherical surface.
To open the program, unzip the downloaded folder, and double click the item with the same name as the folder. Allow for some time to open. If the program does not work, follow the instructions on how to install the program using python code from GitHub.