Thank you for your very fast answer, the problem is solved 🙂
Just an another question, the image used in the result are my big thumbnail post (https://www.cookomix.com/wp-content/uploads/2016/04/mc-flurry-thermomix.jpg)
I didn't see an option to specify a custom size for the_post_thumbnail (like medium, full, large etc… or a custom size defined in wordpress) i didn't activate the image resizing in the plugin to save processor and because i have already the right size generated. There is a way to call programatically a custom size for the_post_thumbnail ?
Thank you !