A simple observation, but one that sounds useful and which I had never heard of before.
It's a littbe bit buggy though, in some cases the highlighted/inspected element's margin and padding are calculated incorrectly and the screenshot is cropped. Maybe I should file a bug report.
Double check that you select the correct element. Yesterday while testing, I noticed that I was selecting a child node and loosing some padding and background color so I needed to move up and select the correct node to take the correct screenshot!