To add photos to your photo stream on your PC, set up iCloud on your PC and enable photo stream as explained here: http://www.apple.com/icloud/setup/pc.html. Then to add photos from your computer, open Windows explorer, select Photo Stream under Favorites on the left sidebar:

Click on Photo Stream, then to the right double-click on My Photo Stream. Click on Add Photos on the top the menu bar:

Select the photos you want to import, then click Open.