I must say I'm extremely pleased with this answer from Gigabyte. It is clear that, with the only proof being some pictures taken with a cell phone, Gigabyte wasn't obliged to replace anything, specially, as Iñigo explains, existing so many steps in the RMA procedure.
Having said so, that's why I'm extremely grateful to how the company has handled this case, relying on the customer. So a very big thanks, specially to Iñigo (who has thoroughly followed this case) and to runn3R and Dark Mantis for being so helpful.
Just one thing, about the comment Dark Mantis makes regarding the "responsibility check" through the RMA process. In this case, there was this kind of check in the shop I bought the part! I signed a receipt from the retailer when I delivered the mobo for repairing. It says the mobo is under warranty and that it doesn't boot (along with more details like my data, the model of the mobo, the things I gave like cd, wires, manual, etc.). And there is a specific field for stating the visible damages and it is empty (because there wasn't any). When I showed it to the shop they said that, even with that document, they wouldn't change the part because the damage would have been done by the manufacturer. However, they didn't do anything to prove it, leaving the customer (me) alone.
Honestly, I expected a similar answer from Gigabyte. However, they have preferred to trust me, which clearly shows the difference between a big company (from which I'll stay as a loyal customer) and a company (the shop) that let their customers down (and from which I'll stay as far as possible).
So, in short, many, many thanks for your help and trust, Gigabyte.
P.S. I'm going to edit the first post to show how well Gigabyte managed this case.
P.P.S. I've replied your mail, Iñigo, with my data. I hope my next post here will be from my own computer!
Edit: Ops, it looks like I can't modify the first post nor my previous messages. However, I can edit this one. Maybe there is a time limit or something?