Yes Pat, the grey "frame" will be brightest.
Still, Brad's observation is correct, thanks for it Brad, i missed that.
So, Pat, once you've selected several files, the default marker will be the first you've selected, as i previously said BUT if you click on a different SELECTED image you make that one the marker.
So, basically, looks like is not necessary to start the selection with the correct one, is enough to click on that correct one after you've selected all, no matter in which order you've done that selection.
Just give it a try and remember, all you need to check before hitting the Sync button is that marker image is the one with most brighter frame.
PS: I've made a mistake in my previous post. For random selection you hold CTRL and click , not ALT+SHIFT as i said, sorry for that...For mac users obviously is CMD click.