How to edit a screenshot
(Editorial note: This User Tip uses the Preview version included with Mavericks and earlier versions of OS X. For Yosemite and later OS X versions please read How to edit a screenshot.)
If you want to post a screenshot that contains information you don't want to appear on this site, you can use OS X's Preview app to delete or obscure that information prior to posting it. No special software is required.
For example: you took a screenshot of an email message such as the following, but you don't want the sender's name to appear when you post it to Apple Support Communities.
Open the screenshot on your Desktop by double-clicking it or selecting Open from the Finder's File menu.
If that action does not cause it to be opened with OS X's native Preview app, control-click (or right click, or two-finger tap) the screenshot, and from the contextual menu that appears, select Open With > Preview.
Preview will open with the screenshot as in the following:
Select the Edit tool (circled):
The editing toolbar will appear with a variety of tools from which to choose. One simple tool is the "line" (circled in the screenshot below). Select it.
Selecting the "line" tool will cause another icon to appear, which enables you to choose from a variety of line thicknesses and arrows.
Draw a line by dragging your cursor over the text you wish to obscure:
Save the edited screenshot by selecting Save from Preview's File menu. The edited version will replace the original one.
Insert it into your question or reply using the "camera" icon that appears above the text entry field.
Drag the screenshot to the Trash when you no longer need it.
Other Preview tools are available for entering shapes, text entry fields, and image editing. You can use any number of them to erase or obscure personal information.