When you add an asset to the Upload Library it shows you that the public URL is something like this:

• http://public.workbooks.com/=QzM/logo.png

Sometimes you want to access these over https, for example to avoid getting browser security warnings due to some parts of an https-accessed web page being downloaded over http.  To convert this to use https simply change the first part of the URL so it's like this:

• https://secure.workbooks.com/resources/=QzM/logo.png