You can delete photos from your Photo Stream in iPhoto 9.2.2 or later, or on any iOS device with iOS 5.1 or later.
When you delete a photo from your Photo Stream, the photo is removed from the Photo Stream view in iPhoto and automatically deleted from the Photo Stream view on any other devices and computers that use the same iCloud account and have the required software.
Important: Any Photo Stream photo that was downloaded to your device while it was running iOS 5.0.1 or earlier must be deleted manually.
Deleting a Photo Stream photo removes it completely from your Photo Stream. To add a deleted photo back to your Photo Stream, you need to upload the photo manually.
To delete a Photo Stream photo in iPhoto:
Select Photo Stream in the Source list on the left, and do any of the following:
Select the photo or photos you want to delete, and press the Delete key.
Select the photo or photos you want to delete, and choose Photos > “Delete from Photo Stream.”
Control-click the photo or photos you want to delete, and choose “Delete from Photo Stream” from the shortcut menu.
Drag the photo or photos you want to delete to Trash in the Source list.
Photos that have already been imported from Photo Stream into Events in your iPhoto library will not be deleted from those Events.