No I don't think that you are missing anything, it is the wiki that is missing what I had considered a basic requirement.
However, on going away and thinking about it I fear that it might be an intentional decision by Apple. They have also removed the alignment options for images, i.e. images can only be centered in a page.
My guess is that they have decided that wiki 3 is not an image library and therefore have not included the option to upload multiple images!
I have looked into the upload .js code and because of the information that the wiki returns to the page it's not obvious how they would handle the creation of lots of images in one go 9 OK anything is doable but I fear the way they have implemented the image addition doesn't make it easy)
All in all not a very helpful response but I wouldn't hold your breath for .zip uploads to become available.
In some ways it would be easier to install an image gallery 'app' on the server and use an iFrame on a wiki page to look at it from within a page.