CleanShot is a small and extremely basic screenshot app.

How basic? This is your workflow. Click the "Capture" button; move the mouse cursor to the top-left corner of the region to capture; left-click, hold and drag to highlight the capture region; release, and hit Enter to confirm.

Once you've done that, by default the program both saves a screenshot to your chosen folder, and copies the image to the clipboard (either of those actions may be disabled from the Settings dialog).

This worked for us, but we did notice one major issue. After you've clicked "Capture", CleanShot lightens the screen, and then restores the original colour of a region as you select it. That's great with pictures, but a total disaster if you're drawing on a white or pale area as the highlighting is invisible.