Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Release IOS 5.1 Beta 2 To Developers
Earlier today Apple released IOS 5.1 beta 2 to developers with the build name of 9B5127c.One of the biggest feature in the update is the ability to delete individual photos from your photo stream
Photos taken using iOS 5.1 can be deleted from Photo Stream on your device and will be removed automatically from Photo Stream on your other iOS 5.1 devices. Older photos can be manually deleted from your iOS 5.1 devices.