Using Rhino, turn display mode to Rendered as shown in the image
Click on the button labelled Materials on the right toolbar
Click on the + to import your image and choose Picture from the drop down list
Select the image that you wish to use as your texture (Use the images that you have photoshopped or use the ones in the sample package) Successful import will result in the left image
Repeat this until you have uploaded all of the textures you wish to use
Return to the Grasshopper Window and locate the component Custom Preview, as shown below
Place your cursor over Material and right click it..
Select the texture you want to use for this particular category.