I am probably the wrong person to be helping because I seldom edit pictures in my word processing program. I use a graphics program (currently SnagIt) for that.

However, the first link is to instructions on using the Crop tool in Word 2010.
Please go to that page:
http://office.microsoft.com/en-us/wo...010355164.aspx

Look for the section on Crop to a Specific Shape.
In the drop down under the Crop button you will see the choice to "Crop to Shape." If that is chosen, you are given a choice of shapes.

The alternative is to first insert the shape and then fill it with the picture.