Jon, sorry, I should have replied sooner. In fact, I did look into the Canon software. It seems that ZoomBrowser has that function, but the software that's specified for my camera is ImageBrowser (a replacement for ZoomBrowser, I guess). When I read the documentation for ImageBrowser (on the Canon web site), it no longer mentions the ability to upload photos to the camera.
I did, however, have a long chat with Canon tech support. They said that if a photo has not been modified, it could be copied back to the SD card, provided that the file structure on the card hasn't been changed. Well, when I look at the icons for all of the photos that I took in portrait mode, they show up sideways on the computer (however, when I actually open them to view, they're right side up). So I took the liberty of righting the icons using the JPG lossless rotation option in Irfanview, which corrected the orientation of the icons without changing the actual viewing orientation. But that's apparently enough to make them unrecognizable to the camera. Also, I have a sprinkling of panoramas that I put together on the computer, that won't play when I put them on the SD card.
But as a test, I took all the original photos (the 600 that I wanted to show) that had no editing, and put them back on the SD card. When I tried to look at them in the camera, the little green led started flashing, and I had time to view the first 4 or 5 before the camera shut down (a repeatable problem).
So I guess I've wasted enough hours trying to get this to work. I have a couple of other options. I can connect my laptop to my TV via HDMI cable, but the photos don't seem to have the same "pop" to them (color saturation, I guess) as they do from an SD card. I can also show them on my smaller (and newer) 32" TV, which has a direct SD slot, but that would mean that I have to carry the TV into the living room when I have company. Or I can try the WD TV Media Player, if I can bring myself to shell out the bucks to buy it.
I'm still weighing my options.