Here’s a neat little trick that lets you overcome the iPhone/iPad’s deliberately limited pinch-expand zoom. It lets you zoom into images right down to individual pixels.
Start by opening the picture you want to zoom in on. Tap the edit button, as indicated below…
… then tap the rotate/crop button. Tap to rotate the image 90 degrees, then tap Done.
Now repeat all the above, but this time rotate the image back to the way it was (that is, you’ll need to tap the rotate button three times). Tap Done.
You will now be able to zoom into the image via the pinch-expand gesture, and will be able to keep going all the way down to pixel detail – although the pixels will be blurred because of the way iOS handles zoom.
Note that this only works for as long as you have the picture open for viewing. If you close it, and then open it again, the usual zoom limitation will be applied again and you’ll have to repeat these steps if you want pixel-level zoom.