I was testing with cover images at my native Wikisource, but I couldn't produce a single preview with an image. Then I found the real issue: the PageImages extension isn't installed in Spanish Wikisource at all!

Not believing my eyes, I went to my local Wikipedia, and they have it installed. The same with my local Wiktionary, my local Wikiquote, even my soon-to-be-defunct local Wikinews, they all have it! Wikimedia Commons, of course also have it, same as Wikispecies, and even Wikifunctions (although I dont really know what images would be displayed). Incubator? You got it. MediaWiki wiki, check.

Look in contrast, English, French, Chinese, Arabic Wikisources: none has it. From my non-exhaustive search Bengali Wikisource is the only one I've find with the Extension on.

I hope the issue and the solution to this is self-explanatory.

Just make sure when setting up the $wgPageImagesNamespaces variable, to include the main namespace as well as the index.

Related: T323570