The second parameter (second item in each of the parentheses) is the version of the button to display after the event (the mouse rolling over the first image) has taken place, and so, onMouseover ...

The image probably isn't loaded by that time. I mean, it's probably a couple milliseconds, if that, between when you assign that src and when you ask for the width and height.

It is called Lightbox JS and it basically uses JavaScript to open up a neat larger view of the image on the page. I use it on many sites, but you can see a quick example on the RustyBrick Mobile ...

I think Google is now ignoring the "static" version (before the javascript runs) and just evaluates the "final" (rendered) version. For articles, I embed resized images that link directly to full ...