Default size of the big image in the slideshow: 756x250px, but the image should be bigger than 756x567px.
Why? When you create your feature image, you have to keep in mind that it will be displayed at 756 x 250. That is the big image you have in your slideshow. However, doing this, will cut your image on the small preview-menu in the slideshow.
Best is to create images at 756 x 567. This will keep the 4:3 ratio in the menu, and your image will be displayed as usual. You just have to center what you want to be in the big image on the slideshow.
For the full width slider images use 1008 x 756 px as featured image size.
If you checked these points, all should be fine now. If not, maybe you found a bug...