Hi @poppyanthology,
I hope you are well today and thank you for your question.
You can try regenerating the thumbnail images by using the following plugin.
To display the post featured image as it is without cropping it, please use the custom plugin shared in the bottom of the following topic.
https://colorlibsupport.com/t/featured-images-and-slider/#post-55214
Best Regards,
Movin