Hi Zinnia,
Your picture "library" probably resides in iPhoto, and I'm guessing that you sync your iPad and iPhone to one of the two Macs? If so, that would be your "master" library. You can sync all of your photos from that library onto your iPad and iPhone by simply selecting "Sync All Events" on your Photo tab for each device.
For Mac to Mac "syncing", however, you have to copy the files from your "master" library to your other Mac. You can do this by copying that library to a Flash Drive, Hard Drive, or by connecting the two Macs together using FireWire and then copying the one library to the other Mac.
Once you get the "master" library onto your other Mac, you can keep the photos updated on both of them by attaching the device that has the new photos you are uploading (whether that be your iPad, iPhone, or an actual camera) to each of them in turn, and uploading the photos to each Mac. Just be sure that you have all of the iTunes sync information turned off on the second Mac, so it doesn't try to open iTunes or sync your device to iTunes on that Mac when you are just trying to upload your new photos to your "backup" library.
Does that make sense?
Cheers,
GB