Reply To: Slider Images Not Centered

I always recommend using a plugin called Regenerate Thumbnails whenever changing or adding new image/thumbnail sizes. This plugin will automatically adjust your image sizes to fit the size you have specified immediately (instead of having to wait for the adjustment). Download and run this plugin anytime you add or make changes to your images.

Once you give this a try, let me know if it fixes the issue – http://wordpress.org/extend/plugins/regenerate-thumbnails/