Image width

All the images I’m loading into the slideshow are 649w x 432h. They load in fine, but I noticed that there was a gap on the right of the images. So I used ‘inspect element’ in Chrome, and it looks like the width of my images is defaulting to 628w for some reason and I can’t fix it. What’s going on here?

http://ww.wp.xz.cn/plugins/slideshow-jquery-image-gallery/