Always good to have a good article on this very useful trouble-shooting function. I think I saw something about getting rid of the thumbnail thing in a recent update to Cocktail. I couldn't imagine why anyone would want to eliminate that feature. I very much like the floating (for a moment) thumbnail. It makes it so easy to get to the new mark-down editor.
I'm not sure how the new screen capture functions if one takes several in a row or a second one before editing the first, etc. Perhaps one can get to that editor another way. 
Even if you don't catch it in time, though, you can find the image in the Finder and annotate from there. Either right click on the file, choose Quick Actions and then Markup, or click to select an image, then press the space bar to get a Quick Look.
While Quick Look is on screen, you'll find a Markup button at top right.
Frankly, I tend to use Monosnap from the Menu Bar because I can never remember the correct secret handshake code!
But Apple's new editor is much better and powerful than Monosnap's.
command-shift-5, period.
"When all else fails...
read the directions!"
