This workaround will work until Apple fixes the Windows software issue
You need to add at least one photo uploaded to Photo stream from a Mac first. If you don't have a Mac, login to iCloud on your PC, then either borrow a Mac for two minutes, go to an Apple Store or any other store that sells Macs
1. Make sure you are logged into iCloud on your Windows PC and Photo Steam is enabled
2. Find a Mac (Borrow from a friend or a retailer. You only need 10 minutes or less to do this).
3. Go to System Preferences (The gears icon) or you can select System Preferences...from the Apple menu at the top left of the screen
4. Click on iCloud and log into your iCloud account (uncheck the syncing options)
5. Open iPhoto
6. From the "Share" menu at the top of the screen, click iCloud
7. Click Use iCloud
8. Click on Photos in the toolbar at the left and drag a photo to iCloud under the heading "Shared" in the toolbar
9. Stop sharing and Log Out of iCloud on the Mac (see instruction that follows)
IMPORTANT: When you've copied an image to your iCloud Photo Steam, DON'T FORGET TO STOP SHARING FROM THE MAC AND LOG OUT
a. From the iPhoto menu within iPhoto, select Preferences
b. Click on the iCloud icon in System Preferences
c. Uncheck the "My Photo Stream" box
d. Now go to System Preferences (The gears icon) or you can select System Preferences... from the Apple menu at the top left of the screen
e. Click on iCloud and log out of your iCloud account
IF YOU LEAVE AND FORGET TO LOGOUT! 😮
Change your Apple ID password ASAP
https://appleid.apple.com/
Back at your PC, you need to click on iCloud Photos in the Windows tool bar, then right-click on My Photo Stream and choose "Add photos..."
If you try to add them by dragging to the directory location, as was previously possible with the iCloud Control Panel, it does not work.
NOTE:
You cannot add photos that are already inside the "My Photo Stream" directory by using the "Add photos..." file open dialog. It will add them, but in a very inconsistent way. For example, I tried clicking "Add photos..." and selected 12 photos that were already in the "My Photo Stream" directory, but only 5 were actually added to My Photo Stream and iCloud for Windows only showed the first one, of the 12 images I added. However, when I moved the images to the desktop and then added them using "Add photos...", they all appeared in Photo Stream as expected.