Magnifying JPEGs viewed in Safari?

Just downloaded a gorgeous 2592 X 1944 pixel JPEG of an equally gorgeous 
Alpine scene, which opened initially at browser window size

However, the pointer, when moved over the picture, turned into a little 
magnifying glass which, when clicked, re-centered and magnified the 
image within that window. blowing it up to what I guess was "a one 
screen pixel per image pixel" magnification.

Haven't found any mention of this neat capability in Safari Help.  Is 
this a Safari-specific feature?  Does it happen automatically, or is it 
something that needs be coded into the image source, or into the HTML 
code that puts the image on a web site?  Are there ways to accomplish 
the same thing in non-web viewing of images, e.g., using Preview or 
other image-viewing applications?