Currently you can’t delete individual or select groups of photos from your iCloud Photo Stream, which is rather unfortunate. You can however backup photos that are not currently in your Photo Stream, if you use iPhoto 9.2 or newer on your Mac.
I didn’t discover this by reading any documentation, but rather by stumbling over this out of sheer curiosity. I figured I would try and drag and drop photos from my ‘Events’ Library into Photo Stream to see what happens. Low and behold, this works and iPhoto will immediately start syncing those photos back to iCloud.
When I dragged my photos over, I made sure to batch-select all photos grouped into Events, however when I checked my Photo Stream, it didn’t seem to preserve the Events. So what ends up happening is Photo Stream becomes just a massive bucket where your photos are stored. I was really hoping for a better solution from Apple.
iCloud is proving to be a great syncing and backup solution that is transparent to the user. You don’t really have to think about what’s going on your iOS device(s), as all the magic just happens in the background. Apple still has a lot of work to do on iCloud though, and I’m sure they are already hard at work on improving and adding functionality to it. On a personal side note, one thing I still want to see is it syncing videos from the Camera Roll. You can voice your feedback to Apple by going to apple.com/feedback/icloud. I know I did my part already, and suggest you do yours.