Can I use Microsoft clipart in or on my book?
Remember that a lot of the clipart found at websites is illegally published. You should never, ever use clipart, or any other image you randomly find on the Internet, without finding the real copyright holder, and obtaining a written, legal release from them.
Most of the clipart you find online is also of poor quality, is not 300 dpi, and not designed for print publication. You don’t want to use something that’s going to be too dark, too light, fuzzy, or pixelated in or on your book. One bad image can ruin a reader’s perception of the entire book.
Regarding specifically using Microsoft clipart (which can also be under 300 dpi, so be wary of that), your question is answered in detail here.