Hm, getting always stranger: I clicked (in Sigil) "Tools"->"Convert to standard Sigil file", and the appearence looked all well, i. e. the assigned fonts appeared as well as the images. But when I clicked "Remove unused media files" those are suggested for removal. And when I run epubcheck, now the message is:
Code:
Col: 65: ERROR(RSC-007): Die referenzierte Datei "OEBPS/images/W3Ccss.png" wurde im EPUB nicht gefunden.
(as example, for it's the same with other images. The font, however, has been accepted).
Translation: The refernced file ... has not been found in the EPUB.