Indeed in the storyboard view, the thumbnails of video and still images are not centered. This looks a bit weird for smaller images/ Unlike in the online manual were they appear to be more centered. (I wonder how they manager to get a screen shot like that.)
Double clicking on a thumbnail selects the clip or image, playhead will be at the start. This also works in the "normal" timeline mode, at least in my version (22.0.2126.0),
but not always....
try is not always reproduceable): go to storyboard; select a clip one mouse click; switch to timeline mode and select another clip, one mouse click; now the new clip is selected but the playhead is not always set at its beginning; after a double click is flaw is undone and a slingle clikc is sufficient to move the playhead to the start of the selected clip
Must all be an error; I suggest that you inform customer support about this.