Currently only <img src="..."> is recognized. Responsive images allow <img srcset="..."> as well as <source srcset="...">, both of which could contain multiple URLs. isImagePreviewTarget and isPreviewable in markup.js look like a reasonable place to start updating this logic. After that it's probably necessary to chase the getImageData implementation that's called from _getPreview.
This has already been filed under bug 1064715.