Stop slides being cropped

Hi,

I am using metaslider and I don’t want the slides that are a slightly different size to be cropped. Unfortunately for this project I can’t just resize the images with a background color in play.

For example, the slider is 500px x 350px. Most of the images are those exact dimension. Some are 500px x 300px. How can I stop them from being stretched out and not displaying the whole image?

https://ww.wp.xz.cn/plugins/ml-slider/