The clipart you find in Office 2007 is licensed to you by MS. It is safe and free to use without license issues.
Like anything else on the internet you have to be careful where you go for files. Although it isn't common there has been malware associated with image files, ie:
https://www.google.ca/search?q=malware+jpg&ie=utf-8&oe=utf-8&gws_rd=cr&ei=8pBhV9rkBomeyQL8m6DACg
There are a number of relatively safe sites that provide free images you can use as clipart.
Open Clip Art - Free
http://www.worldstart.com/open-clip-art/print/
Free Web Images - Free Graphics
http://webdesign.about.com/od/freegraphics/?nl=1
5 Awesome Downloadable Free Graphics
https://community.articulate.com/articles/best-of-2014-5-awesome-downloadable-free-graphics
http://www.articulate.com/rapid-elearning/how-to-fine-free-stock-photos-clip-art-powerpoint/
Bing Image Search
http://www.indezine.com/products/powerpoint/learn/picturesandvisuals/bing/
Learn how you can search images using Bing's amazing image search functions.
Author: Geetesh Bajaj
Date Created: August 1st 2013
ShapeChef – Addon ClipArt Collection and Store Your Own
https://www.shapechef.com/libs/graphics-and-templates
ShapeChef provides access to a growing collection of high-quality PowerPoint graphics, charts, and icons.
Free Stock Photo Sites for E-Learning
https://community.articulate.com/articles/free-stock-photo-sites-for-e-learning
Looking for some fresh images to use in your e-learning projects but short on budget dollars? You’d be surprised at how many stock photo sites offer free images you can use in personal and/or commercial projects.
Four ways to find free images to use legally
http://www.techrepublic.com/article/four-ways-to-find-free-images-to-use-legally-with-google-apps/
The article is written specifically for Google Apps, but can be adapted for MS Office also.
www.indezine.com/ is a good source for Free clipart and PPT Templates (and tutorials)
PowerPoint Tutorials -
http://www.indezine.com/products/powerpoint/learn/
Subscribe to their newsletter to get notice of free templates and clipart and links to PPT tutorials
This site has a weekly (Mondays) newsletter that offers you choice of 6 free downloads. They can be various types of products that are normally sold on the site: clipart, background images, photoshop addons, templates to name just a few.
The only disadvantage is that you have to put up with additional advertising emails during the week.
2015 11 10- How to Easily Search through Free Stock Images
http://blogs.articulate.com/rapid-elearning/how-to-easily-search-free-stock-images/
A Complete List of Free Stock Image Sites for E-Learning
http://blogs.articulate.com/rapid-elearning/complete-list-free-stock-image-sites-e-learning/
http://www.articulate.com/rapid-elearning/little-known-ways-to-create-your-own-graphics-using-powerpoint/
http://www.articulate.com/rapid-elearning/how-to-create-characters/
September 30th, 2014
http://blogs.articulate.com/rapid-elearning/customize-free-stock-photos-presentations/
August 11th, 2015
Art Text Lite (free) - Create WordArt like text and SmartArt like graphics - Win8 Metro App
http://www.techrepublic.com/blog/window-on-windows/quick-tip-create-professional-looking-graphics-with-art-text/7858
Takeaway: Create professional looking, high quality graphic headings, logos, icons, and buttons in Windows 8.
By
Greg Shultz June 12, 2013, 10:47 AM PDT
http://www.articulate.com/rapid-elearning/little-known-ways-to-create-your-own-graphics-using-powerpoint/
You used to be able to go to Office.com to get clipart from MS, but they shut down that site back in 2013. Here is the "official"Reason / Excuse for the change - Technet Changes in Office 2013 Page
https://technet.microsoft.com/en-us/library/cc178954%28v=office.15%29.aspx?f=255&MSPPError=-2147217396
https://technet.microsoft.com/en-us/library/cc178954.aspx?f=255&MSPPError=-2147217396