There are some extra things you need to do in Sigil 0.6 to get a cover image to display as it did with earlier versions. Otherwise the thumbnail will not appear, or the cover will be duplicated or appear once, but badly cropped. This particularly applies when converting to .epub and .mobi with Calibre.
This is covered by the new user manual for brand new documents but the solution below applies to a document I started in an older version of Sigil and then modified with 0.6
All names are case-sensitive and 0.6 uses .html not .xhtml now.
Go to Book Browser view;
In the "Text" folder make the first file "Cover.xhtml"
Right click on this and look at "Add Semantics",
make sure that "Cover" is ticked.
In "Cover.xhtml" insert the image inside a header tag e.g. <h2 title="Cover"><img alt="" src="../Images/cover.jpg"/></h2>
In the "Images" folder make sure you have added "cover.jpg"
(This
must be called "cover" but can be .gif or .png or .jpeg, as appropriate)
Right click on this and look at "Add Semantics",
make sure that "Cover" is ticked here as well.
Open the "content.opf"
Add this tag to the metadata <meta content="cover.jpg" name="cover" /> just before the </metadata> tag
.epub books will open at the cover. It will also make the cover visible in the TOC for many .epub readers (not iBooks for the iPad) .mobi readers can use the "Go to" function to find the cover. This is as close as you can get to make any Kindle or Kindle app start at the cover.
Further info. "Cover" is a reserved word in the epub spec and for the Kindle. You can only use it to describe the outside front of a book. It is therefore not seen by some e-readers as Content, because it is not inside - unless you tell it that it is. Calibre does this by renaming the Cover.xhtml as titlepage.xhtml but it leaves the filename and the id (name) as "cover" so .epub readers still open at the Cover.