.sow-slider-base ul.sow-slider-images li.sow-slider-image .sow-slider-image-container {
    position: fixed;
}

You currently have it set to “absolute”.

To add CSS:

• If you are using WordPress 4.7 or later, use the “Additional CSS” option in the customizer.
• If your theme has a custom CSS option, use that to add the CSS shown above.
• If not, install the plugin Simple Custom CSS.

Learn to use the Chrome Developer Tools to help you see and test changes to your CSS.

You can achieve this by applying following CSS code.

.sow-slider-base ul.sow-slider-images li.sow-slider-image .sow-slider-image-container .sow-slider-image-wrapper {
    margin: 0;
    z-index: 2;
    -ms-box-sizing: border-box;
    -moz-box-sizing: border-box;
    -webkit-box-sizing: border-box;
    box-sizing: border-box;
}

https://i.is.cc/2iO5IVAy.png
https://i.is.cc/2iOkeJLT.png

Hope this will help.

Thanks.