I played with some earlier versions of calibre in virtualPC to look at .opf files and the contents.
Save book with windows calibre 0.5.14, reference to description and cover in opf file. Add book with windows calibre 0.6.29 and cover and description successfully imported.
Save book with windows calibre 0.6.20, reference to description and cover in opf file. Add book with windows calibre 0.6.29 and only description successfully imported. The cover reference is present in the opf file but the name is missing the "_number"(I assume this is the database reference number).
Maybe this will help to identify my problem.
thanking you again.