congratulation to Weasel1984 who made me discover a flaw in my image hosting system: if you upload two files with the same name but in a different case, it is considered as a different one as on Linux systems the names are case-sensitive
i.e. these are two different images:
http://pics.imcdb.org/1306/MBW1231.jpg
http://pics.imcdb.org/1306/MBw1231.jpg
but I can't backup that properly because on Windows I can't have two files with the same name, even if the case differs... I'll have to find a workaround for the backup... or simply force all names to lowercase