Please set $FillMode option, 0: streach, 1: contain, 2: cover, 4: actual size, 5: contain for large image and actual size for small image.
The value 1 or 2 or 4 will be good choice to keep aspect ratio
<script>
jQuery(document).ready(function ($) {
var options = {
$FillMode: 1, //[Optional] The way to fill image in slide, 0 stretch, 1 contain (keep aspect ratio and put all inside slide), 2 cover (keep aspect ratio and cover whole slide), 4 actuall size, 5 contain for large image and actual size for small image, default value is 0
$AutoPlay: true, //[Optional] Whether to auto play, to enable slideshow, this option must be set to true, default value is false
$DragOrientation: 3 //[Optional] Orientation to drag slide, 0 no drag, 1 horizental, 2 vertical, 3 either, default value is 1 (Note that the $DragOrientation should be the same as $PlayOrientation when $DisplayPieces is greater than 1, or parking position is not 0)
};
var jssor_slider1 = new $JssorSlider$("slider1_container", options);
});
</script>
Reference: Jssor Slider Options