Is the Program Monitor set to full resolution?
Is under that little colored button (when you hover over it).
One possibility is that the image is larger than your video frame size. If you're scaling a high-res JPEG down to DV resolution, it won't look as sharp.
My hi-rez jpeg's look sharp in a dv timeline.
But not as sharp as an original would at 3000 x 2000. Sometimes people see that loss and ask what's wrong, not realizing it's normal.
Let's try to narrow things down a bit.
What are the pixel x pixel dimensions of the JPEG(s)?
What is the Sequence's Frame Size?
Are you using any Scaling, either with Scale to Default Frame Size in Preferences, Scale to Frame Size from the Project Panel, or with the Fixed Effect>Motion>Scale?