After you gave this idea to me, yes. I tested in Google Chrome (my main browser), Firefox and Internet Explorer. It works fine in IE and Firefox. Thanks for giving me this idea, I think it should be because of Comodo DNS (My Chrome is modded, its called Comodo Dragon) so I wil try changing it to the standard DNS and see what happens.
edit: I've discovered the problem (its not even about DNS, I changed it to a standard one months ago and forgot about that), its the protection "Do not allow websites to know where you came from (suppress HTTP Referrer header)". If you turn its ON pixiv doesnt let you see their images, I thinks its because they dont know that this request is coming from their own site, bad programming if you ask me, since its the first site that acts like that.