well, what was my problem?
An huge collection of photos organized in galleries, in a static website.
So i pick all and upload in wp, but you know and i had sadly discovered: for every img loaded wp make 3-4 version in different size downto 150x150px. And all are in the same upload directory. What is 10000 photo uploaded in an ONLY, huge, folder? a nightmare, unusable folder. A workaround will be put a publish date BEFORE any img upload, but custom upload dir solve simply a question that will be a standard wp feature.