You can pretty much use any image you'd like as a grain. When you click on "New Painting" from the "File" menu, you'll see an option at the bottom left that says "Grain". If you click on that circle another window opens with a selection of different grains. Beneath the selections there is a button that reads "Load a File...". Click the "Load a File..." button and select the image you'd like to use as your grain and click "Open". The image/grain will appear in the "Grain" circle for use in your new painting.
Nothing is easy to the unwilling.