Which version of Word for Mac are you using? The steps might be a little different.
If you are using Microsoft Word 2008 for Mac, here is what to do:
In your open Word document, look at your menu bar and click Insert -> Picture -> From File... to insert your image.
Make sure your Formatting Palette is open. If not, click View -> Formatting Palette in the menu bar.
With your image selected, go to the Formatting Palette and see if the Picture category is revealed. If not, click on Picture to open it.
There should be a Crop button. Once clicked, the edges in your image will change to show handles you can drag to crop the image.
Once done, click the Crop button again to finish the crop.
Hope this helps.
BTW, I usually like to use Mac OS X's Preview (in Mac OS X versions >10.5) to get more powerful cropping and editing.
Not sure about "delete the cropped sections of all images in a document to save file space".