Choosing "Cover photo" for the article.
For now, there is only an option to "report" the cover photo (the big one in the header) as unsuitable or disturbing. Perhaps, there should be an option to choose the cover photo from those related to the article by means of voting, or otherwise.
It is not possible to suggest a cover photo. Click any image to open the gallery, then click the “pin” icon in the top-left corner.
Alexander Fainshtein commented
You said "It is NOT possible to suggest a cover photo" -- but you meant "It is NOW possible to suggest a cover photo" :-)
I was reading an article on Wikiwand and then a screen shows up "Suggest as cover photo Would you like to suggest this photo as the cover photo for this article?" with two buttons. "Yes, this would make a good choice" and "No, never mind". Behind the that is a dark grey screen with NO photo and when I click on the no button it opens a new tab showing the same thing.
My guess is that there is a bug in the program. I have version 3.1.