We suggest you to use images with proportions around 16:9 … 16:8 For example, 1040*585px, e.i. (1040px/16)*9=585px. But you are always able to use “Preserve default slide proportions” option for the slider:
We recommend to make images 1200-1600px on the bigger side; 66-76% JPG quality – this is a good compromise between image size/quality and page loading speed.
Alternatively for Chocolate theme, you can edit “Supersized” slider settings in /dt-chocolate/js/scripts.js line 43. Set “1” value there: