The Background Tab of Edit Options looks like this:
There are two primary modes you can operate in are
To use a Colored Background, uncheck "Use Graphic Background" and use the "Set Board Color" button to set the color. Both the sample screeen (with the pieces on it) and the graphics sample (on the right) will change to show the new board color. You can use the regular colors shown, or create your own.
When you press the "Set Board Color" button you will see the standard color dialog shown below:
To use a Graphic Background, check "Use Graphic Background." You now have to decide if you want the graphic transparent (which means that the lower-left hand corner color will be transparent) or whether you want it normal. You also have to choose a graphic, and decide how you want it shown.
Press the "Set Graphic Background" button to choose a graphic for the background. For now, you can only use Bitmap graphics. This will likely change in the future to allow JPG and GIF graphics, at least.
You now have to decide whether you want to "Draw" the graphic (which simply puts it in the upper right-hand corner of the game board), "Center" it (which centers it), "Stretch" it, which stretches the graphic to fit the area of the board, or "Tile" it, which repeats the graphic over and over again over the whole area of the board.
As soon as you choose how you want to draw the graphic, it will be reflected in the sample screen. When you press "OK," or "Save" it will be done to the entire board (not including the MiniBoard).
On certain types of graphics, it might be desirable to have your background color show through. If that is the case, you can check "Transparent Graphics," and then the lower-left hand corner pixel on your graphic will determine the transparent color for the graphic. All pixels of that color in the graphic will "disappear."