In the United States, the answer depends on how much leaked information you've received and what you mean by "reposting."
The First Amendment protects further commentary on the leaks.
If you mean talking about the leaked information, sharing bits of the leaked information, or publishing commentary about the leaked information, you're right in the heartland of First Amendment-protected activity.
A similar issue arose in Bartnicki v. Vopper, 532 U.S. 514 (2001), where a radio host played and discussed an illegally intercepted recording of two union officials talking about contract negotiations with a local school board. The union officials sued the radio host, but the Supreme Court said that "a stranger's illegal conduct does not suffice to remove the First Amendment shield from speech about a matter of public concern."
So if we can treat the cards, their leak, or Wizards' response as a matter of public concern (and there's a good argument for each), then it seems clear that you're free to discuss, write about, and even publish portions of the leaked content, as part of your commentary.
Copyright law prohibits publishing too much of the leaked material.
But if you're talking about a wholesale republication of the leaked material, you're in much more dangerous territory. Those materials are protected by copyright, which means you are generally not free to make or publicly display copies of them. "Fair Use" is, of course, an exception, but the complete republication -- especially outside the context of commentary on newsworthy events -- is probably not fair use.
In Harper & Row v. Nation Enters., 471 U.S. 539 (1985), a magazine obtained a leaked copy of President Ford's memoirs and published an article with about 300 words's worth of quotes lifted directly from the book. The book published sued for infringement, the magazine asserted a fair use/First Amendment defense, and the Supreme Court ruled for the publisher, holding that the "generous verbatim excerpts" took the article outside of fair use.
In your case, then, the question would be how much of the leaked material you could copy or publish. It would be tough to argue that you can just blast out copies of the leaked material in full, with or without accompanying commentary. But even publishing small portions could be problematic. In Harper & Row, the Court noted that the answer depends not on the quantity of material copied (300 words from a 200,000-word book, or 0.15%, in that case) but the "qualitative nature." Because the magazine copied material about Ford's mental process surrounding his pardon of Nixon -- "the most interesting and moving parts of the entire manuscript" -- the Court found that the substantiality of the copying weighed against a finding of fair use.
I don't know Magic well enough to say, but it's conceivable that publishing images of specific cards, classes of cards, or other materials could be more problematic than others, while publishing images of the backs of the cards or their packaging would probably be less controversial. (Again, all of this assumes the copies are happening as part of broader commentary on the issue. Outside that context, we're probably just talking about a routine copyright violation.)
Bottom line: You should feel free to discuss or describe or the leaked information, but the more of it you copy or publish, the farther from safety you roam.