For what it’s worth, the 1200×514 image is consistently resized to 582×520, regardless of file format (jpg/gif/png). I have never had any thumbnails set to that size, and it resets to that size regardless of my media thumbnail settings (which are all currently set to 0). I am baffled :/
what’s showing: http://www.acrosstown.org/wp-content/uploads/2016/01/WP-Slider-HK-1216-582×520.gif
what should be there: http://www.acrosstown.org/wp-content/uploads/2016/01/WP-Slider-HK-1216.gif
Just for shiggles, I installed “display all image sizes” plugin to see if that would provide any insight. No image size of 582×520 was detected.