There are many excellent websites out there catering for the interests of the butterfly enthusiast and photographer. Below are links to a few sites that I have found particularly useful, or just especially enjoyable to visit. I shall add additional links in due course.
Roger's website sets the gold standard for photography, and contains a wealth of information on French butterflies, including many valuable observations, tips, and keys to the identification of difficult species. I regularly refer to this site.
Philippe's knowledge of the butterflies of France and Corsica is very extensive, and his website includes numerous images covering almost every species on the French list, including many of the rarest. Burnet moths are also covered, along with some interesting natural history observations. Well worth a visit!
Guy's website provides a wealth of original observations, photographs and information, predominantly but not exclusively focused on Switzerland, and includes coverage of early stages and distribution. Diary entries and sightings pages cover 20 years of records. A unique resource!
Butterfly Conservation's European Butterflies Group promotes the conservation and study of Lepidoptera across Europe. The website contains much information on butterflies, on a country by country basis, along with details of important contacts, recording forms, and many other useful resources.
There can be very few people who have seen and photographed more species of European butterflies that Matt Rowlings. His superb website is a vast catalogue of excellent photographs, including some very rare sightings, along with helpful field-notes and much additional information. A regular and invaluable point of reference for me over the years.