A good collection is he http://www.weltderwolken.de/

BTW, the best book about cloud classification which I know is "The
Cloudspotter’s Guide" (by http://www.cloudappreciationsociety.org). It
is really easy to read, but surprizingly exact. You my or may not like
the style, but it is the first book I've seen which takes you by the
hand and walks you through the cloud classification system, so that even
I finally think it's pretty obvious. The pictures are black and white
only, too, but it really doesn't matter in this book.