iMovie 6.0.3 timeline conundrum: adding photo without losing sync

How does one add a photo to an existing video with audio in 6.0.3 so that it simply replaces, for example, 5 seconds of the video WITHOUT moving the rest of the video? IOW, I don't want the video or audio to move at all, just for the inserted image to replace the 5 seconds of video at that place. I can't seem to do it without the video shifting 5 seconds. I have extracted the audio and tried every combination of locking audio and dragging the photo to no avail.

Obviously it's a simple thing, but I can't find any reference to this in any literature! : ) Thanks.

Rollo