Select the picture you want to work with, and then on the Formatting toolbar, click Edit Pictures.
In the Edit Pictures task pane, under Edit using these tools, click Color.
Under Hue and saturation settings, type -100 in the Saturation settings text box or move the slider all the way to the left.
Note: To undo an edit, click the Undo command on the Edit menu. Edits to pictures are not saved until you save them.
If you're using Office 2010 or later, see Change the color of a picture to learn how to change picture color, saturation, or tone.