The library show only the picture of the course, not of individual lessons. I remember that earlier the course picture was shown only when there was no individual lesson picture.
@VeraI - Would you be able to link me to a course where this is the case?
It is not only in the course. It is the same in the lesson and the playlist view.
Example Course: Login - LingQ
Some examples for lessons with individual pictures in this course:
WN#001, Plätzchen backen, Login - LingQ
WN#002, Lebkuchen und Spekulatius, Login - LingQ
WN#004, Die Weihnachtsdekoration, Login - LingQ
WN#005, Unser Adventskranz, Login - LingQ
WN#006, Der Adventskalender, Login - LingQ
WN#007, Nikolaustag, Login - LingQ
etc.
@VeraI - Thanks, I’ve added this to the list and we’ll look at getting this fixed!
Thank you Alex. I know this is for sure not on the top of the priority list but I want to let you know about it. And as long as it not works it makes not much sense to add lesson pictures 
I remember that it has worked. Than there was an issue that the picture of the lesson was shown only, and not the course picture if the lesson has no picture. Maybe when you fixed this you created this bug instead.
The logic behind must be:
If there is a lesson picture show this, if not show the cource picture. If this exists neither show the LingQ standard lesson picture.
@VeraI - Absolutely, thanks for letting us know about it 