By default OS X saves screen shots with a shadow around them. This can be easily changed so that screen shots are saved without shadows.
- Open Terminal (Applications > Utilities > Terminal.app).
- Write or copy
defaults write com.apple.screencapture disable-shadow -bool trueto the Terminal window and press Enter.
- Then restart SystemUIServer by writing or copying
killall SystemUIServerto Terminal and press Enter again.
If you change your mind you can reverse the process by changing
false in the first line and then restarting SystemUIServer again.