Open the Utility folder, inside the applications folder, and launch Disk Utility.
Select your HD in the LH column and click on Repair Permissions.

Once completed, try to launch one JPG and see if it made any difference.

The fact that the other account is unaffected would normally mean that the software or system was not damaged.

By the way, OS X has a series of versions from 10.1 to 10.4. It could made troubleshooting a little easier if you were to mention what flavor of OS X you are running.