I am not sure whether adding a link to a winning photo
would be useful
at all, especially with long filenames or filenames written in
non-Latin alphabets (or even in Latin alphabets in countries which use
different ones); just imagine how long would typing those from your
keyboard take!
Yes, it could be painful, but providing access to the original files is
the only way to allow properly reuse of the photos.
An option would be to link to a central page instead of each of the
images. «You can view and download the design of this calendar and each
of the individual photos at
http://commons.wikimedia.org/wiki/Commons:Wiki_Loves_Monuments_2012_calendar
»
What's wrong with QR codes? Each sheet in the calendar has two sides
and one stays unused.