Normally, an image of the library blocks should be displayed...
I now use firefox instead of chrome. When I noticed the problem in firefox, I checked if the problem was happening in chrome: the prewiew was showing in chrome... Later, as of this post, I go back to chrome so I can do a printscreen : the preview is no longer displayed in chrome!
Could this be an intermittent problem?
I tried to reload the page: still the same problem...