Hi Santi, this isn't the first time this question has been asked on this list--I have given a thorough answer to it earlier this month (see http://lists.wikimedia.org/pipermail/wikilovesmonuments/2012-September/004346.html).
In order not to have to answer that question again :-) I have just added it to our FAQ at https://commons.wikimedia.org/wiki/Commons:Wiki_Loves_Monuments_2012/FAQ#GLAM_category.
Thanks,