Thank you everyone for your help.
This was supposed to be a quick task. I had 30 minutes to put photos on the iPad to then take it away show the photos. Being unaware of the limitations, this went south. So the opportunity (and the urgency) is passed. I am going to try all this out at my leisure when I have enough time.
To summarize what I have learned about moving photos iMac to iPad.
Have to wire iPad to Mac via USB.
Can't use iPad like a removable device and add/remove files via something like Finder on the Mac iPad. Must use iTunes.
Impossible to keep photos that are ONLY on iPad when you "sync" (except for Camera Roll).
iPad can only be synced to ONE computer. I think this also means to ONE user account on that computer.
To do what I was thinking.
Copy all desired photos into one iMac user account.
Import photos from iPad into that same account.
Plunk all the desired photos in one folder (may have subfolders) and sync that folder to the iPad using iTunes.