For images loaded via AJAX, or that I don't want indexed, use the data-* attribute approach:
<img data-src="path/to/image.jpg" class="js-lazy-load" />
<img src="path/to/image.jpg" />
But for images in the HTML that I do want indexed:
<a href="path/to/image.jpg" class="js-lazy-load">Image alt text here</a>
<img src="path/to/image.jpg" alt="Image alt text here" />
Seems that would preserve index-ability and the intent of the page (for accessibility) without affecting SEO (hopefully). But would love a second opinion.
Edit: Any feedback on how this approach - page with links to images versus page with inline IMG tags - would compare in regards to page rank. I'm guessing the inline IMG tags would fare better since each outgoing link would detract from the overall page rank (unless they had rel="nofollow" which would be counter productive).