Good overview about the patterns currently being discussed:

Recently, all of the ongoing discussion around responsive images just got real: a solution is currently being discussed with the WHATWG. And we’re in the thick of it now: we’re throwing around references to picture and img set; […]

The markup pattern that gets selected stands to have a tremendous influence on how developers build websites in the future. Not just responsive or adaptive websites, either. All websites.

Naturally I’m in the picture camp, this just makes most sense to me.