Sony DCM-M1 Operating Instructions Manual page 63

Changing the index image
To change the index image during playback or playback pause
(1) Press INDEX CHANGE on the Remote Commander at the scene you want to
make into the index image.
The confirmation message appears.
(2) Select YES using B /b , then press ENTER.
After the PLEASE WAIT message, which appears while the change is written
in the disc, the camcorder resumes playback.
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