again, TIFF files are not supported by most browsers unless you have
installed a proper plug-in to handle them .... is like trying to open
PDF files in your browser, you may be requested to either download
them or install the reader plug-in. Learn more here:
http://en.wikipedia.org/wiki/Comparison_of_web_browsers#Image_format_support
fancybox is not a browser window therefore it doesn't necessarily
handles plug-ins as your browsers do. If you can see your TIFF files
while linking them directly is because you have the a plugin installed
to render them properly.
Don't get me wrong but unless you have a good reason of using TIFF
files within your web pages I would rather convert them into formats
that are widely supported by most browsers (JPG/PNG/GIF). Another
thing to consider is that you have no control over the visitor's
browsers and they may not see what you are expecting them to see ....
and users get annoyed when they are prompted to download or install
something in order to see an image