I am clearly too paranoid. I was looking at my screen thinking what sort of information leakage there would be uploading a screenshot and then 'trimming' it to remove the stuff that I didn't want out there. And then thinking the upload sends the whole image, then the tools crop it and you get a 'final' version. But the original? Still there somewhere. Not that this is an original thought mind you, XKCD famously parodied the problem of creating a web site requiring email and an password and noting that lots of people use the same password everywhere so you could harvest passwords that way.
On a Mac, you can get a cropped screenshot on your clipboard using Command+Shift+Control+4. Then just drag the crosshairs to choose an area to capture, or press Space and then click a window to capture.
You can save a screenshot to the Desktop, and optionally edit it before uploading, by not holding down Control, with both the 3 and 4 shortcuts.
