Before taking other action, it might be prudent here to rebuild your iPhoto Library database. It is possible that the photos are there, inside iPhoto's database, but because of the znomaly you experienced while in iPhoto, the pointers within the database might be corrupted and unable to show you the photos inside the iPhoto program.
There are two ways to rebuild the iPhoto database.
(1) While opening iPhoto, hold down the option and command keys. You can ask it to rebuild portions of the database, but I would recommend rebuilding everything, by checking all the boxes. Since this rebuild acts directly on the iPhoto Library, you should first make a copy of the iPhoto Library for safekeeping in case you are not satisfied with the outcome of this procedure.
(2) Download iPhoto Library Manager, a very powerful program (costs $20, but well worth it) which has a more thorough and superior rebuild iPhoto library capability than the one that Apple provides. Also, iPhoto Library Manager doesn't change or touch the original iPhoto Library; it leave sit alone but scours through it for all existing photos and creates a NEW iPhoto Library based on all the photos and info it can find in your current iPhoto Library.
I recommend (2) over (1). Make a copy of your iPhoto Library if you go with (1).
This may restore your "lost" photos in the iPhoto Library database. If it doesn't, there are other troubleshooting steps one can take.
Why has this happened with this particular import? Hard to say, but perhaps one of the photos was damaged and iPhoto is having trouble handling it; or perhaps there was a memory or electronic glitch or small power surge in your computer at the moment that the iPhoto database (IPhoto Library) was being updated which corrupted one of its pointers, and the database will continue to give you trouble until you rebuild it with (1) or (2) [preferably (2)].
I'm going to go with 2. The fact that it was invented means I'm not the first schmuck this has happened to. Wish I had paid the 20 bucks the first time it happened and got all those hours of my life back, the 2nd and 3rd times I downloaded and edited. Thank you very, very kindly. I will let you know how it works out.
I ended up taking my macbook pro to the genius bar at the Apple Store in Cerritos. I was told my problem was "not common"...the first step was to rebuild my thumbnails (as Steve626 suggested above). This took the better part of 3 hrs...no exaggeration. Then rebuilding "caches" took another hour. There was one more rebuild which took about 30 mins, after which my Genius did some other steps. The photos never reappeared and the folders with the grey palm trees remained empty. However, after the 3rd rebuild, my Genius noticed that my iPhoto Library contained 125GB of photos. She was shocked. She said that's why the rebuilds took so long. She said the iPhoto Library functions fine until you reach about 40 to 50 GB of data, after which you need to create a 2nd Library. By that estimate, I should have made over 3 libraries by now. This corresponds with Steve's advice as well, since the library cant "handle" new downloads once it is a certain size. The process of moving photos is a bit cumbersome, but I have begun it. Steve626, thank you very much for your insight and expertise.
I think it is a good idea to work with several smaller Libraries versus one big one, but actually, I have read that iPhoto works with up to 250,000 photos and some report they have more than that in their Libraries. I think many users have Libraries well over 100 GB if you read these forums, without issues (my daughter has one that is 170 GB). The iPhoto Library is actually a package-type folder itself, and all the photos inside are sorted into individual folders basically by date, so one is basically looking at many folders used to hold many files here, nothing that a Mac can't handle. Creating a new iPhoto Library makes yet another new folder to contain more folders with photos in them. However I think it makes logical sense to have multiple iPhoto Libraries when one has many photos, tens of thousands, say.
By the way, 250,000 photos with 10 MB per photo equals 2,500 GB, or 2.5 TB of photos.