The use of iconography is very pervasive in presentations, infographics and data visualizations. The minimal, visual portrayal of concepts is quick and easy to understand and is often more effective than photos or complex illustrations. Below are many different sources of icons for your projects.
The Noun Project has a searchable library of over 1 million royalty-free icons submitted by designers all over the world. All icons are FREE to use if you give the designer credit in your final work, or the inexpensive NounPro plan ($39.99/year) gives you unlimited royalty-free licenses to download and use the icons. Plug-ins are available for direction integration with Microsoft Office, Adobe Creative Cloud and Google Docs applications.
The AIGA has made the complete set of 50 passenger/pedestrian symbols developed by AIGA is available for all to use, free of charge. Signs are available here in EPS and GIF formats. This system of 50 symbol signs was designed for use at the crossroads of modern life: in airports and other transportation hubs and at large international events. Produced through a collaboration between AIGA and the U.S. Department of Transportation (DOT), they are an example of how public-minded designers can address a universal communication need. First set of 34 symbols was published in 1974, and received a Presidential Design Awards; 16 more symbols were added in 1979.
Pictofigo images or in other words "Freehand Drawings" are now available to be downloaded from the Pictofigo website and to be used on making blogs, making presentations, developing project/process workflows, adding value to forums, contributing through articles etc. Free and paid plans are available.