Image not centered in box

I had been using the Image Shadow plugin to create a nice, soft frame around my images, and I'd turned off the default border in the theme. Then I realized the plugin was caching the images, and it was adding about .3 of a second to the front page load-time. So I deactivated it.

Now I want the border back, but the default produces an odd effect. The image is flush left, with a right padding area. I want it centered, and can't figure out how to get it that way.