By popular request, I moved the zooming actions to Gwenview status bar and introduced a zooming slider, as can be seen on this shot:
The nice thing about this change is that it removes quite a few buttons from the default toolbar. I am still not sure whether I like it or not, and would like to prettify the "zoom to fit" and "actual size" buttons. I think the button icons should be more distinct: the only difference between the current ones is a 5x5 pixel square.
I will probably play with it a bit more later. Maybe going as far as implementing the mockup mentionned in this post by Jos.