One of the interesting things you can do with Numbers '08 is place an image behind your worksheet's cells. Used correctly, this could add visual appeal to reports you create. Used incorrectly, it could make your work unreadable, so use this hint with caution.
In Excel, when you placed an image, it always resided above the worksheet cells. If you wanted those cells to appear above the image, the only real solution was to make the image translucent. The problem with this approach, however, is that you cannot select the cells with a click, as that will instead select the image (as it's the frontmost object). With Numbers' flexible layout, whereby you can place grids anywhere you like on a blank screen, this isn't an issue, as you can set the layer for grids and images separately. I thought it'd be fun to try putting an image behind a table, but my first efforts failed. When the image is placed behind the table, the table covers the image -- even if the Graphic inspector's Fill setting for all the cells on the table is set to None.
After some stumbling around, I found the cause and the solution.
The reason that nothing seems to happen is that the table itself has a fill value of solid white. So even though the cells on the table may be set to not fill, the table itself is filled, and that's what you'll see "behind" the not-filled table cells. So to make your background image show through the table, set the table's fill to None. To do that, click on the area to the left of the "A" and above the "1" to select the entire table. Then go to the Build inspector and set the fill to None. Bingo, now your picture is visible, and yet in the background, so that the mouse selects the worksheet's cells, not the image.
You can also use a Color Fill with the Opacity slider set to something lower than 100%. However, I prefer to use None, because it provides more flexibility. Why is it more flexible? Because once you have the table's fill set to None, you can now select individual cells (or ranges of cells) within the table and set their Fill options independently, and those fill settings will affect the visible background image:

Mac OS X Hints
http://hints.macworld.com/article.php?story=2007081405330694