Your image can either be one that you've already saved to your computer, or you can annotate a screen clip. I will provide instructions for the latter. For the former scenario, you can simply open the desired file and then immediately go to Step 4.
For those taking the screen clip route:
 Arrange window as desired. Type in Command + Control + Shift + 4. Select the desired area.
 Open Preview.
 Type in Command + N. This will paste your screen clip into a new file.
|Can you recognize whose half face that is?|
 Go to Tools > Annotate and then select which option you are looking for--adding an arrow, oval, rectangle, or text.
 Change the options using the buttons at the bottom of the window. You can change color, line thickness, etc.
 Save your edited image. Go to File > Save or simply type Command + S.
Ta-daa! You're done.
Note: For those who are interested in blocking out certain text in your image using Preview, there is an easy way to do this! All you have to do sufficiently increase the line thickness icon before drawing your rectangle. Then, when you make the rectangle, it will appear like a solid rectangle as opposed to the default (and sole option) no fill rectangle.
Here are some images to aid you:
|The original image|
|Add a rectangle using a thicker line.|
|Draw your rectangle over the text you wish to omit.|