I saw it somewhere in one of the forums/threads where a screenshot was posted with a highlighted area while the rest of the screenshot was dimmed. Does anyone know if it can be done in OS X or is a third party application required?
1) Take your screenshot using the standard keyboard shortcut: CMD-Shift-3
2) Open the resulting file using the Preview application (the file should be saved to your Desktop)
3) In Preview, select Tools -> Annotate
4) add the desired highlights and save