The best way is to use Menucool Ninja Slider that allows any sized images, and that is also responsive.
If the aspectRatio still doesn't fit your requirements, you can consider adding your own element as the ninja slider element with whatever background-size setting that you prefer. For example:
<div style="background:url(/img/3.jpg) no-repeat 50% 50%; background-size:cover;"></div>
You can set aspectRatio option to be "auto" in the ninja-slider.js
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