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Danaus Plexippus
NoelSal - This was indeed a rare sighting for Spain, but it's not impossible. Monarchs have been noted in a variety of of locations outside of North America. See this map at Wikipedia for an example:
http://en.wikipedia.org/wiki/File:Monarc...
Did you take this photo in the wild in Spain, or in a butterfly house? If in the wild, this is a very rare sighting for your area.
I was reading your bio. There's nothing wrong with the quality of your images. Phones can do wonders these days. My only advice would be if you are not sure of the species of your spotting, then check the 'help me ID this' box when posting so others can try to help you with ID.