How to use fill method on prefab object?

Hey, I want to create a 3D cube with each face having a pie chart. I believe that the easiest way to do this is to use the "fill method" found in the "Image (Script)" component. A prefabricated cube could then be made out of 6 faces, and each of these faces could be scripted to have wedges change in proportion through the fill method. However, I can't seem to get the source image (a sprite) to display on the cube while still giving me the option to use the fill method. Could anyone help with this?