Can you use PowerPoint images on your website?

Yes, you may.
I contacted Microsoft support and they confirmed this.
However, I don’t know how much trust I would put into a chat representative for legal concerns. So, I’ve provided some more information…

Here is an excerpt from Microsoft’s own website:

Clip Art and Sample Art

The End-User
License Terms that accompany your
software describe the permitted
commercial uses of images, clip art,
animations, sounds, music, shapes,
video clips, and templates that
accompanied the product. Find End-User
License Terms for Microsoft products.

The Clip Art and Media gallery
provides a compilation of artwork. See
the use terms for the description of
permitted uses. If those terms do not
meet your needs, our Clip Art partners
at Office Online provide a variety of
images you can license directly.

In the absence of language to the
contrary in the License Agreement,
Sample Art (which includes images
customarily found in the “sample”
folders within Microsoft operating
systems) may be used for personal use
only. You may not sell, lease, or
distribute Sample Art, or any
materials you create that use Sample
images, for any commercial purposes.

Additionally, here is an excerpt straight from PowerPoint 2007 EULA:

Media Elements and Templates. You may
copy and use images, clip art,
animations, sounds, music, shapes,
video clips and templates provided
with the software and identified for
such use in documents and projects
that you create. You may distribute
those documents and projects
noncommercially. If you wish to use
these media elements or templates for
any other purpose, go to
www.microsoft.com/permission to learn
whether that use is allowed.

You may find out additional details here.
For any further inquires I was suggested to contact microsoft directly, in writing, at:

Microsoft Corporation

One Microsoft Way

Redmond, WA 98052-6399