How can I transfer photos from the Camera Roll to Photo Stream?
Any photos that exist in the Camera Roll that didn't come from the device itself had to come from another device. The primary method for getting these pictures into iCloud's 1000 picture buffer is to enroll that other device into iCloud.

You have one case of a picture that was taken either more than 30 days ago or 1000 pictures ago or you deleted your entire Photo Stream and want to start over. In this case, you may need to sync those photos to a computer or another device using the Camera Connecton Kit to inject them into your current Photo Stream.