Here is an alternate method to add photos to Photo Stream from a PC:
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.
Additionally, ensure your photos are in the right format:
Photo Stream supports JPEG, TIFF, PNG, and most RAW photo formats. Photo Stream does not work with video.