Um, you do realize a magazine is editorial and not commercial usage right? You realize editorially you can sell the images to whoever you want as long as the zoo rules only forbid commercial usage right? There seems to be a major lacking of understanding what is commercial and what is editorial where you have rights given to you under the first amendment in the United States.
The only issue I see is no payment, that means no usage - PERIOD!! You're not paying = you get nothing, it really is as simple as that. Credit means nothing, it won't get you in the door, won't help your career, it's about as big a myth as trickle down economics, big foot and sexy vampires.