how do i upload existing photos on my ipad to icloud?

If you mean to photo stream, you have a couple of options:

1. Open the photos one at a time and take a screenshot. This will add a new low resolution copy of the photo to the camera roll, which will then upload to your photo stream;
2. Email the photos a few at a time to yourself, then save the attached photos to your camera roll. This will add a copy of the photo at full resolution to your camera roll and upload to photo stream;
3. Use an app like PhotoSync, which will allow you to wirelessly transfer copies of all your old photos to your computer all at once. Then delete these photos from your camera roll and use PhotoSync to transfer them back from your computer to your camera roll. These will be the original resolution and will upload to your photo stream. (You need an app like this because iTunes will not transfer photos back to your camera roll, where they must be in order to be added to your photo stream.)

Another solution is to share a "Shared Photo Stream Album" with your self.