You want to use the model viewer script, or just images (screenshots) of the models created with the model viewer?
I think this official reply from Wowhead admins back in 2010 on the topic of embedding the model viewer onto third party websites is still valid:
Malgayne wrote:
Armorylite and 3DArmory both are using our script with credit, but without permission. We had intended to keep bandwidth costs down by not allowing other sites to use our modelviewer, but we failed at simple security checks. :P
Naturally the next stop is to politely ask the other sites to stop using our modelviewer in this way—but considering that the functionality is so cool, we're not doing anything like that until you can do the same thing on Wowhead. :)
...
Perhaps I wasn't clear—this is not a behavior we approve of. We've allowed it to continue because we haven't made it available here on Wowhead yet, but once that happens we'll be tweaking the system so it won't work anymore. We'd prefer you did your modelviewing here on Wowhead. :P
http://www.wowhead.com/forums&topic=73401/3d-model-viewer-embeddable
As for screenshots of the models shown in the model viewer, I'm pretty sure that's Blizzard property, not ZAM property. ZAM owns the model viewer, but Blizzard owns the models shown in the model viewer. Blizzard's official stance seems to be that everything is okay to use, as long as you don't charge people money for it.
My post is subject to swift and merciless corrections by ZAM officials, so consider this reply a useless "maybe" to your question.