Well the best way would be to add placeholder image with the height of your choice. May be you can try placeholder image (featured image) of 2000 X 500 pixels. By placeholder image I mean a blank image of png format. The main advantage is that it will adjust responsively in different big and small devices.
I see that you have added fixed height to slider image via custom CSS, I suggest you to remove that as it will caused issue in other smaller devices or mobiles.