Load the clip into the viewer, then go to the motion tab and find 'distort'. Open the small triangle to reveal the controls, then use the aspect slider to get the pictures looking right. Then copy that clip, select all the other clips in the timeline and do a 'paste attributes' 'basic motion'.
Of course, it could also be that you're exporting an anamorphic sequence, which is supposed to look like that because it's the TV that stretches it to the correct proportions.
Oh, actually, rereading your post it probably is that. Because you're exporting a still, not a sequence, so the settings must be anamorphic somewhere. You could possibly load your exported pic into QuickTime 7 and use the video>display function to change the aspect.
Open in QT 7, go to Window>Show Movie Properties then go to the Video Track tab > Visual settings > uncheck 'preserve aspect ratio' and type the numbers you want into the scaled size boxes. I have no idea what your frame size was or what your photo size was, so I can't tell you what numbers to use.
Then 'Export' > "movie to picture' and use the options to get back to jpeg.