This is really cool. How are errors handled though? What if you have a transient error to 1 of 50 images -- does that bork the whole download? The user could get a corrupted file.
I feel like from a UX perspective, it'd be ideal to be able to give some friendly error message to at least acknowledge that the failure is on the server end. A page that says, "Sorry, we're having trouble accessing your files right now. Please try again in a minute.," seems more user-friendly to me than a download that suddenly fails with no explanation. Nevertheless, this is very cool.
[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html section 14.35.