Thanks for your help, Dark Mantis. I'll contact runn3R.
I understand that Gigabyte should have "good faith" in this case, because, as you said, it could be inferred the picture was taken before the scratch. However, why should I risk my warranty by scratching the mobo? Who scratches a mounted and working mother board in such a place (just next to the outer box)? Even my seller, before sending the part, give me a receipt showing there was no visible damages in the part.
I feel a little bit defenseless in this case. I hope Gigabyte can give me some explanations about this RMA.
Thanks again.