Portrait mode image background. Doesn't work properly

When i resized my browser window to Portrait mode (i have high resolution monitor, and i don’t need to resize my browser in landscape mode) the background image stretch right by height, but cut by width (white bg appears at the right of screen).

This issue may not be so critical, IF the same problem appear on mobile devices. iPhone and iPad don’t display bg image properly – white background appears on right.

Please advice.

https://www.ads-software.com/extend/plugins/site-background-slider/