There is an easy fix to get a good aspect ratio for your pictures, using jssor.
Open jssor.slider-22.1.9.min.js or the file you embed in your html page.
Search for “$FillMode:0” and set it to $FillMode:1 or $FillMode:2 or $FillMode:3.
The default $FillMode:0 will not keep a good apsect ratio.
Feel free to test the values.
$FillMode:2 for example, creates a center/middle crop for the image, keeping the aspect ratio.