French law states that publishing the image of a work of art (architectural in this case), without the permission of the rightful owners, is considered a crime. To do this legally you must wait until 70 years after the death of the artist. According to the French intellectual property code, violators are liable to a fine of €300,000 (about £260,000), and three years in prison.
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So since it was constructed in 1889, enough time has passed for us to be able to take a picture of the parisian monument, without having to fork out 300 grand. But what about the lights that it displays at night. Introduced in 1985, they too have been copyrighted. The 280 spotlights and the 20,000 lights that make it flicker every hour, are considered as a work of art.
Photographing the Eiffel Tower when it's illuminated could actually land you in a lot of trouble, even if you aren't using it for commercial purposes. For each image you post, you should, technically, ask for permission and pay copyright fees. C'mon then, whoever's guilty, stick your hand up!