I posted about this to the upstream forum ( http://foro.pixjuegos.com/viewtopic.php?t=224&start=0 ) and got some replies.
The good news is that the upstream developers posted a link to a tarball containing the source for the images. http://pixjuegos.com/descargas/recursospixbros.tar.bz2
The bad new is that the source is in the form of PSD files, gimp opens them and recognises the layers but the text is uneditable and misses the white border making it practially unreadable ( see http://fotolog.pixjuegos.com/galimg/1213781898.jpg ). The background of the bottom section also looks much darker.
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