The Pieces Tab of Edit Options looks as below:
There are four basic shapes that your pieces can take, squares, circles, diamonds and stars. You choose which you want to use in the game here. Squares are the default shape. If you change the shape, it changes in the color area to the right, and also on the sample screen on the Background tab.
You can also choose what six colors (out of a palette of 9) you wish to use in the game. Just pick the color name from the drop-down list. The color will be instantly changed in the color sample next to the color name.
Note that you are NOT allowed to have any two colors the same, i.e., if you've already picked red for one piece, you cannot pick red for a different piece. Note that if you change the color, this is also reflected on the sample screen on the Background tab.
Well, custom graphics aren't supported yet. But very soon (by the Gold release, for sure), you'll be able to use custom graphics which will be available on my web site for use in the game.