I'm assuming you have a PC for this. Go into your search bar and enter "snipping tool". After it's open go into your vault and click on "New". Now, just highlight the areas you want to be in the picture.
Nilex on SMP9 gave me this link, it works a treat for editing screenshots to be smaller before you even take them: http://app.prntscr.com/
I don't use these much I like to stay on SMP3 Very colorful (Forgot to move my mouse when I took a screenshot...)