Is there any way to crop larger?

[eluser]Chad Crowell[/eluser]
I am uploading and resizing an image to 315x315, which is working fine. As expected, when the image is not square it is resizing to keep the aspect ratio, so something like 315x280. This is great.

After that happens I would like to crop the image to 315x315, so essentially it would center the image vertically, and add white background to the top and bottom to make it square, and then save it.

Is this possible?
How?