My original iPad has images that my 2 newer devices do not have on iphoto.
How are the images stored on your original iPad? Are they in the camera roll, in events downloaded using the camera connection kit, or are they in albums synced to your ipad from a computer?
If the photos are in the camera roll or in events downloaded from a camera, you can transfer them to your iMac and MBA by directly connecting the iPad using USB. Then you can download the photos to iPhoto like from a camera.
Only, if your photos have been synced to the iPad from a computer, you will have to save them first to the camera roll; then you can download them by connecting the iPad via USB.
What IOS version has your iPad?
For new photos you might consider using Photo Stream to transfer your photos between the devices.