I'm developing a jQuery plugin that works on top of the DOM structure (html). This plugin will be to improve page performance (improving the user experience).

The images are only loaded when scrolling the page (like LazyLoad http://www.appelsiini.net/projects/lazyload/enabled_timeout.html).

Note: two good examples are the use of LazyLoad youtube (suggested videos) and facebook (photo album)

But the data-original="" attribute is not accessible to all Searchers. And HTML5 does not work in all browsers.

This plugin will work in older browsers so I need an alternative to HTML5.

I believe that the best alternative is to use javascript
<A href="image.jpg">
and convert to
<IMG src="image.jpg">

If I do this:

<a href="http://site/image/image.jpg">Alt content</a>
Is the same as:

<img src="http://site/image/image.jpg" alt="Alt content">
The two ways are equivalent for SEO?

Note: This question is not about Javascript