http://support.apple.com/kb/PH2606
iCloud: Add photos to Photo Stream manually
You can manually add photos from iPhoto, Aperture, or the Windows desktop to your photo stream. You may want to do this if youd like to add photos that dont appear in your photo stream automatically.
Note: On a Mac, you can manually add photos even if you selected Automatic Upload (when you set up Photo Stream in iPhoto or Aperture).
From iPhoto or Aperture
Open iPhoto or Aperture (whichever application you set up to work with Photo Stream).
Drag one or more photos from your library (Events, Photos, Places, or Faces) to your Photo Stream; the photos you drag are uploaded to iCloud and added to your photo stream.
If you have iPhoto 9.4, you need to select photos, then choose Share > Photo Stream.
From your Windows desktop
From your Windows desktop, drag any photo to the Photo Stream Uploads folder you chose in the iCloud Control Panel.
Note: Photos in Photo Stream appear in the order they were originally taken, not in the order they were added to Photo Stream. If you manually add photos, they appear throughout Photo Stream in the appropriate date order.
For a list of file types that can be added to Photo Stream, read What photo formats does Photo Stream support in the Photo Stream FAQ on the iCloud Support website.
http://support.apple.com/kb/HT4486