Select one of the files. Press COMMAND-I to open the Get Info window. In the Open With section select Preview as your default application then click on the Change All button.
This may not stand because as soon as you open some other jpeg file with Illustrator it may revert back because you cannot assign more than one application as the default for the same file type.
Kappy, that did not work. I did a Google search on the subject and learned about an app called RCDefaultApp. I downloaded that and placed it in my Library file as directed. I then closed System Preferences and reopened it. The app showed up on the far left of the "Other" line of System Preferences.
This let me set default apps for each "extension." Sure enough, Illustrator was set to open jpeg, jpg, JPG, JPEG and PNG files. I changed all of them to Preview. Well, it still didn't work. Even though Preview was open and actually allowing me to view one of these files, it would not let me open another one of them.
About ready to tear my hair out, I decided if all else fails restart the computer. That fixed the problem. It appears the opening preferences got returned to their original values. The files now open with Preview.
Interestingly, when I started installing iLife, there was a warning that I would have to restart my computer when the software was loaded. I omitted GarageBand which I don't need. There was never any alert that the software was loaded and I should restart my computer. So, I didn't. Maybe some corruption crept in as a result.
While at it, I also went into Finder preferences and checked the block under Advanced to show all extensions. Quite a few of those census files had none, and they do now.
Preview is working now and is opening all those files. I also tried two small PNGs which I trashed because they would not open, and they work, too. Happiness!!