Would the collage be considered a derivative of the original(s), where it is created based on or making use of the preexisting copyrighted material, or could it be seen as transformative, which builds on the preexisting copyrighted material, and adds an entirely new expression or meaning to the material? It would ultimately be up to a court to decide.
I lean toward roger's analysis and echo his advice that getting permission from the rights holders would be wise.
Artist Jeff Koons (an "appropriation" artist) uses the copyrighted works of others to create his own works, and he has often been sued over his created works as a result. He has lost more infringement suits than he has won. One of the suits Koons won was Blanch v Koons (No. 03 Civ.8026 (LLS) S.D.N.Y, Nov 1, 2005; aff'd No. 05-6433, 2nd Circ, Oct 26, 2006).
The Koons work that was the subject of dispute in the Blanch case was judged by the court to be a transformative work and, therefore, not an infringement on Blanch's copyrighted photograph. Koons had taken a Blanch-created ad from a magazine and used a portion of the ad in his collage.
The case is worth reading as it covers the factors a court looks at when deciding whether a work is a derivative or not, transformative or not, a fair use or not, infringing or not. In fact, all of Koons cases are worth reading as the court discussions on infringement are informative.
At any rate, if a work can be seen as a derivative of a copyrighted work as opposed to transformative, permission from the copyright holders would be necessary. Making derivatives is an exclusive right of a copyright holder.
As a note on Andy Warhol and his Campbell Soup paintings - Warhol could have been sued by Campbell's for infringement, and he could easily have lost the suit. However Campbell's did not sue because they liked his paintings and purchased one for their corporate headquarters.
As another note, the First Sale Doctrine does not apply here. The First Sale Doctrine would apply if Animus' wife were to purchase a magazine and then resell the magazine. It would not apply if Animus' wife takes copyrighted material from the magazine and sells it separately or makes something from the material and sells it (see Mirage Editions, Inc v Albuquerque A.R.T Co, 856 F.2d 1341, 1988).