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 [[ https://es.wikisource.org/wiki/Especial:Versi%C3%B3n | isn't installed in Spanish Wikisource at all! ]]
Not believing my eyes, I went to [[https://es.wikipedia.org/wiki/Especial:Versi%C3%B3n | my local Wikipedia ]], and they have it installed. The same with [[https://es.wiktionary.org/wiki/Especial:Versi%C3%B3n | my local Wiktionary ]], [[https://es.wikiquote.org/wiki/Especial:Versi%C3%B3n | my local Wikiquote ]], even my soon-to-be-defunct [[https://es.wikipedia.org/wiki/Especial:Versi%C3%B3n | local Wikinews]], they all have it! [[ https://commons.wikimedia.org/wiki/Special:Version| Wikimedia Commons]], of course also have it, same as [[https://species.wikimedia.org/wiki/Special:Version | Wikispecies]], and even [[ https://www.wikifunctions.org/wiki/Special:Version | Wikifunctions ]] (although I dont really know what images would be displayed). [[https://incubator.wikimedia.org/wiki/Special:Version | Incubator]]? You got it. [[https://www.mediawiki.org/wiki/Special:Version | MediaWiki wiki]], check.
Look in contrast, [[https://en.wikisource.org/wiki/Special:Version | English]], [[https://fr.wikisource.org/wiki/Special:Version | French ]], [[https://zh.wikisource.org/wiki/Special:Version | Chinese]], [[https://ar.wikisource.org/wiki/Special:Version | Arabic]] Wikisources: none has it. From my non-exhaustive search [[https://bn.wikisource.org/wiki/Special:Version | 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.