After you've created the shared photo stream, click Photo Stream in the Library pane on the left and then double click on your shared Photo Stream. Click to switch to the Info panel on the left (or type W to switch between the panels).
In the Info panel pop-up menu select Comments. You'll see a comment field where you can add your labels for each photo. When you've finished typing, click the Post button at the bottom. These comments will get synced back to iCloud so anyone viewing your Photo Stream will see them when viewing each photo.