[General] Blog image replaced

[Jared Rhine]

The header graphic of blog.chandlerproject.org has been replaced with 
the "Chandler Project" logo graphic.  (It was the rounded blue OSAF logo 
before).

This isn't a perfect fix to making the blog Preview-ready, but it's 
substantially less jarring this way.  Probably most other changes can be 
done as time permits, tracked by bugzilla #8407[1].

[1] https://bugzilla.osafoundation.org/show_bug.cgi?id=8407