Thanks for your help, nayeeem!
Last night I’ve made a change in the ‘tutor-template-functions.php’. Instead of “placeholder.jpg” now is “‘assets/images/curso’.$post_id.’.jpg”. So, we were able to set an image for each ID (it means each course).
However, I don’t think it is the best way to define the course thumbnail.
I’ve seen videos on Youtube where there was an option during the course creation to set the course thumbnail.
Check this video at 12m.19s: https://www.youtube.com/watch?v=dV3lfYVRUV0&t=31s
As I can’t find this option, I wonder if it is available only on paid version of this app.