Zoomable? Right there?
Sorry, I fail to see the point --
There are plenty of third party apps that focus on viewing images, and any one of them is just one or two taps away.
Their developers have put a lot of effort into it, and I'm sure they're better that whatever I could come up with.
These apps run full screen, too, which is kind of important for phone sized screens.
Re: thunderbird, and desktop UIs in general --
1) they typically have larger screens (a 10.1" notebook is considered "small", while a 10.1" tablet is "large") and
2) on those larger screens, UI elements can be smaller, and there can be more of them, because a mouse is more precise than a finger