Is there any way to re-download Photo Stream photos on a PC/Mac? If I accidentally delete a photo from my Photo Stream (or get a corrupted disk or lose a laptop) and later want to recover that picture from the online Photo Stream that has my pictures stored for 30 days in full resolution, how do I do that?

I tried turning the Photo Stream on and off, and it does not download the existing pictures again. If the photos are being stored for 30 days, it would be a natural thing to want to re-download on-demand. The current way it works seems like the photos are stored for 30 days as long as any one registered device has not downloaded, but once all "registered" devices downloads it, it is removed. In this context a registered device is one that was signed in to iCloud BEFORE a picture is taken. Is this correct?
For the PC the way I got it to redownload the photo stream is first go to Control Panel then open up iCloud control panel.

Turn Photo Stream to off and close the iCloud control panel.

Just as a precaution, rename the old Photo Stream folder just to have a backup of what was in there before.  This is up to you.

Then go to C:\Users\yourusername\AppData\Roaming\Apple Computer and rename the MediaStream folder to something like MediaStream-old.

Then go back to Control Panel, open up iCloud control panel and renable the Photo Stream.

If you chose a custom download location for Photo Stream then you have to choose it again.  The photos should start coming in, but just remember it will only grab the last 1,000 photos you have taken.

Thanks.