On my iPad quitting an app and restarting it often helps correct problems like this. Here's how to do that on an iPad:
Go to the Home screen and double click the Home button. That will reveal the row of recently used apps at the bottom of the screen. Tap and hold on the app in question until it wiggles and displays a minus sign. Tap the minus sign to actually quit the app. Then tap anywhere on the screen above that bottom row to return the screen to normal. Then restart the app and see if it works normally.
Can you do this on an iPhone? Let me know.
Thanks very much for the tip. That trick is entirely new to me, but it does also work on the iPhone. I can't say whether it fixed the problem, however, because I went ahead and did a full restore - which did fix the problem. I have no clue why this issue developed, but it appeared to actually be getting worse: initially, I would click on the picture I wanted, it briefly would open, and then I'd be presented with a picture from a diffeerent album, and could scroll through photos in that album;the last time I tried iPhoto, it started giving me all the pictures in my library, in no discernible order. If this happens again, I'll try your approach first, and thanks once more.