The "Pinterest Uploader App for iPad" is free and it works beautifully. When you open it the first time, you must add an account. You put in your Pinterest email and password or use the Facebook or twitter options.
It won't work until you add your Pinterest account as it needs permission to add to your boards.
Then you just use the app to upload images. You have a choice of camera or existing. Pick existing and it brings you straight to your camera roll and albums. Select the picture you want and pin it! Easy!