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If the president already had the power to block release of the photos by issuing an Executive order, and if the new law specifies a procedure by which he can do that without an Executive order, then the new law doesn't actually "vest [the president] with brand new secrecy powers" but, instead, vests the president with a brand new and slightly less embarassing procedure to exercise secrecy powers.
He's been ordered by the court to release the photos, and might not win a Supreme Court appeal. I don't know that an Executive Order at this point would change that.
I think the legislation might satisfy this and prevent the court from ruling against him:
1. When the President acts pursuant to an express or implied authorization of Congress, his authority is at its maximum, for it includes all that he possesses in his own right plus all that Congress can delegate.
JACKSON, J., Concurring Opinion, Youngstown Sheet & Tube Co. v. Sawyer
http://www.law.cornell.edu/supct/html/historics/USSC_CR_0343_0579_ZC2.html