tedster - 8:23 pm on Feb 23, 2011 (gmt 0)
You're right that a sub-folder approach will not improve a browser's ability to run parallel threads, so it won't increase page speed.
That said, page speed is not a major ranking factor - at least not so far. So I can't see how a fine point, such as subdomain versus stand-alone domain, is going to make any difference in Image Search ranking. The subdomain approach does seem like a better idea to me from a branding point of view.