Crop all the thumbnails

Hi !
I have a problem with the plugin.
When I add my images and it creates the thumbnails, I’ll see what it looks like and I have all my thumbnails that are cropping down the photos…
For example, I only see people’s legs.
I can indeed crop the photos one by one but is there a way to configure so that when creating thumbnails, it crops everything up or in the center for example?
Thanks for answer (and sorry for my english!)