Hi @lukaribeiro,
I hope you are well today and thank you for your question.
Please try using the solutions posted in the following topics.
https://colorlibsupport.com/t/slider-images-different-size-grey-thumbnails/#post-73946
https://colorlibsupport.com/t/recent-post-image-not-resized-correctly/
Best Regards,
Movin