I am not a lawyer, but if you dig into the software license agreement you'll find this:
3. Media Elements. Microsoft grants you a license to copy, distribute, perform and display media elements (images, clip art, animations, sounds, music, video clips, templates and other forms of content) included with the software in projects and documents, except that you may not: (i) sell, license or distribute copies of any media elements by themselves or as a product if the primary value of the product is the media elements; (ii) grant your customers rights to further license or distribute the media elements; (iii) license or distribute for commercial purposes media elements that include the representation of identifiable individuals, governments, logos, trademarks, or emblems or use these types of images in ways that could imply an endorsement or association with your product, entity or activity; or (iv) create obscene or scandalous works using the media elements. Other media elements, which are accessible on Office.com or on other websites through features of the software, are governed by the terms on those websites.
So the short answer is that you generally can use them, according to Microsoft. But, that doesn't mean the original copyright owner couldn't come after you still - however unlikely that may be. Note we're only talking about the "stock images" included with Microsoft Office, not "online pictures" which it also offers. For "online pictures" you are on your own for ensuring you are complying with any usage terms. Even Creative Commons images often have attribution requirements that you can be sued over if you don't comply.
Personally I only use images from my subscription with istockphoto.com; that way I can prove I have a license in case of a dispute, and they offer some indemnity protection as well. I lost thousands once from using an image from a "free image" site that wasn't really free, and won't repeat that mistake.