I use a 64 GB wifi Air Stash as well as a 500 GB wifi GoFlex Satellite Pro to store my photos. Put them on either with your computer, not your iPad. You then tether wirelessly from your iPad to either device to see them. Works like a charm.
In short, you need to download from your camera's SD or CF card to your computer then back up to either the AirStash or the GoFlex Satellite. I do this all the time when on the road and have about 360 GB of photos.