Weird resize bug when switching slides?

Okay, so I am trying to edit this theme I am using in WordPress (Arras) and use Nivo Slider because I don’t like the slider that comes packaged with it however, whenever I add the slider into the template file it has this weird issue. When you load the page up, everything is fine, but when you change slides it resizes the image and box that Nivo is contained in. I have tried everything I know of and cannot figure why it’s doing this.
I tested it on my modified version of Twenty Eleven and it doesn’t resize the container, but it does have the weird image resize sometimes.

Site Link: http://yenrac.net/

Please get back to me with this as soon as possible!

EDIT: And Yes, the images are the same size.

https://wordpress.org/plugins/wp-nivo-slider/