GreedyGammon board images are in your program files folder under "pics". The name of the folder corresponds to your screen resolution. If you have a screen resolution that is not listed, you can rename one of the folders or add a new folder with your screen width resolution and copy image files into it from one of the other folders that is closest to your screen size. You can also replace any image in the folders with your own images as long as you keep the names the same.
Players are welcome to upload and share their own themes here.
Update: November 2017- There is a "blender" folder in the main install download. Use that to create your own boards easily. Instructions in the readme.txt file
Basically you can just copy and paste textures that you like and the 3d software (Blender) will render it for you.