How to zoom image in Safari to fit iPhone display

Question: 

I want to quickly zoom an image in Safari on my iPhone. How to do this?

Answer: 

The quickest way to zoom in a picture in Safari on iPhone is to double-tap the image you want to magnify. Tapping an image twice will fit the image to your iPhone full screen resolution. 

To zoom out double-tab the image again. Please be cautious when using this trick on linked images, because most of the time linked images will lead you to another page or to the image zoom itself (e.g. using colorbox image preview script).