LearnPress multiple thumbnails

Dear all,
in order to change the thumbnail of my first course, I had to go into “wp-content\plugins\learnpress\assets\images\no-image.png”, and insert my png file renaming it as “no-image.png”, and this works.
The problem is that since there is not specific button to set that thumb for every course I create, all my courses will take the same thumbnail.
I need to have different thumbnails for each course…

Mine is the free version of Learnpress.

Can anyone help me ?
Thanks,
Alessandra