hi,
itis an anoying issue I faced when I wanted to delete a private photo from the photostream, I figured out this work-around itis a long procedure but it does the job:
1- copy photostream photos to your local iPhoto library on your mac (go to iPhoto app, click photostream, select all photos, drag them to photo folder under library)
2- delete all photos on photostream from your mac by turning off photostream on mac (go to settings, icloud, uncheck photostream)
3- delete all photos on your ios devices (go to settings, icloud, photostream, turn it off, confirm delete photos), ps: this should be done on all ios devices using ur apple ID
4- on your mac iPhoto, go to photo folder under library, now you can delete unwanted photos,
5- delete all photos in icloud (go to icloud.com, login, click your name on the top corner, click advance, click reset photo stream)
6- turn photostream on on your mac (go settings, icloud, check photostream)
7- select all remaining photos from photo folder under iPhoto library and drag them to photostream folder on your mac
8- now iPhoto will upload all selected photos to your iCloud
9- turn photostream on in all your iOS devices (settings, icloud, photostream, turn it on)
now all wanted photos will be downloaded from your icloud account to ur ios devices
as I said itis a long procedure but it does the job
hope it helps