Since you have a fixed-width #mainWrapper at 900px wide, you can know exactly how wide the center column will be.
Thus, you can make one bg with the entire top(TL & TR rounded corners) and another with the entire middle and bottom (BL & BR rounded corners). If you want a border, make the second gif taller than you will ever have a page so there will always be side borders.

Assign the bottom gif to the <td>, positioned at the bottom center, and the top gif to the first <div>, positioned at top center. This will always fill the column from top to bottom.

There is an alternate version of this method using three gifs, including one for the middle bg and side borders that repeats vertically so none of the gifs needs to be tall. That would not work so well for the "keep it full height" requirement.