(from http://www.macgasm.net/2011/05/23/disable-dropshadow-mac-os-window-screenshots/)
defaults write com.apple.screencapture disable-shadow -bool true
killall SystemUIServer
defaults write com.apple.screencapture disable-shadow -bool false
killall SystemUIServer
defaults write com.apple.screencapture location ~/Downloads/
killall SystemUIServer
ipconfig getifaddr en0 #(use en2 for thunderbolt ethernet)
save location tip from http://osxdaily.com/2011/01/26/change-the-screenshot-save-file-location-in-mac-os-x/