What version of Premiere are you using?
Have you installed the most recent updates?
It's a known issue. If you open the JPEG's in Photoshop and save them out again, things should go back to normal.
I am using Premiere Pro 5. I just received an email from Jim and his advice to save the jpg's in photoshop and bring them back worked. However when I went to install updates the Adobe Application Manager told me to close the Adobe Premiere Pro. exe. I kept trying to close the program but the notice told me that it was not closed so I am 4.7% through the update and cannot continue. I was able to close the Bridge exe. but not the Premiere Pro exe. Can you help?
Thank you Jim this worked!