Hello,

Please set the image as the background in the slide settings: https://gyazo.com/328c5a8a18988898be7f0d9d9ead63fc

and let us know if you still have the problem.

When you an image, the image is scaled, and white gaps appear, the image on the background is responsive. Pay attention that there is the option to set different images on different devices, try to upload square proportional images.

If you have any questions please feel free to contact us.

Best Regards