You can easily zoom in the image viewer now. (I even find zooming with the hardware keys easier than the tapping stuff.)

There's 2 problems with zooming:

One is that it doesn't provide the 'perfect' zoom factor for the image displayed. There's fixed zoom factors, and it will most likely happen that zoom factor a isn't enough, but zoom factor b is too much and cuts off some of the image content.
Your proposal ("Double-tap would alternate between actual size and making the image fill the screen") would solve this one perfectly, yes.

The other one is that if you look at a number of images of different size, you'd need to re-adjust the zoom factor with each new image. That's not what you want. (Think of the situation when you start a slide show on the TV, the N900 is next to your TV and you're on the couch.) - The point is that photos should automatically adjust. They do when they're too large (the N900 scales them down), so why not scale them up when they're too small?