How to generate fixed with thumbnails

Hello,

I have the following images :

– 1000x500px
– 2000x400px

I want to generate 500x500px thumbnails. I want the final image to be exactly 500x500px. My source image are not in square ratio so there will be blank space on top and bottom of the thumbnails

See this image for a better explanation :

https://www.noelshack.com/2018-25-2-1529423120-recadre.png

Thank you