Listen. Write. Present: The Elements for Communicating Science and Technology by 
Presentation Skills for Technical Professionals: Achieving Excellence by 
Presenting an Effective and Dynamic Technical Paper: a Guidebook for Novice and Experienced Speakers in a Multicultural World by On Google, you can find a ton of free images but you should be careful of copyright restrictions.
After searching for an image, click on Tools and then on Usage rights. From there, you can pick whichever of the options applies to you for the image you are searching:
