This option was moved to the trigger tab, I think. Page 81 of the manual appears to need updating.
Page 88 of the manual...
Page 103...Note that an alert image cannot be moved, copied, or exported here unless it has an
associated JPEG file created using the hi-res option on the Trigger tab in camera settings.
Recall that an alert image is normally just a database entry referencing an actual clip.
Furthermore, deleting an alert image does not delete its associated video clip.
A trigger leading-edge image is captured when the camera actually enters the triggered state.
This is the “alert image” as it is saved to the database with a reduced resolution unless you
have chosen to save alert images as hi-res JPEG files on the camera settings Trigger page.