With the clip on FCP's Timeline, set an IN and OUT point for the section you want to convert. Go to File->Export->Using QuickTime Conversion. Choose "Image Sequence" from the format menu and then choose the file type (I recommend uncompressed TIFF) from the Options menu. IMPORTANT: Make sure you create a new folder for the images!
FCP/QT will create an image file for each frame of video between the IN and OUT points.
-DH