With smaller screen sizes, icons are becoming an increasingly important element of user interface design. Not everyone of you is likely to be using PowerPoint to design a web app (hey I do), but icons can also be useful in regular presentation design.
I am not talking about floppy disks and other ancient icons we still use, but stylised symbols that can be an effective visual short cut to a category of (pick the appropriate) products, benefits, user problems, etc.
One option is to create your own icons in PowerPoint, set the zoom to 400% and create miniature shapes using shape booleans. Often you can use a quick Google Image search to find inspiration for your icon.
Stock photo sites sell endless amounts of icons, but there are compatibility issues when using them in PowerPoint or Keynote. Most icons are design as a vector graphic in Adobe Illustrator. It makes them infinitely scalable (like a PowerPoint shape), but PowerPoint does not read this file format. Copy-pasting Illustrator objects (if you have the software installed) is unpredictable and results in a shape that is hard to manipulate (changing colour for example).
Cropping icons out of a JPG or PNG file is not a solution either, if you forget to compress the small image file, you end up storing a huge image file with all your icons multiple times on a PowerPoint slide. Cropping also kills the vector-like scaling of icons, and background transparency.
A recent trend in web design might provide a solution: custom icon fonts. Modern successors of Zapf Dingbats (what?) provide clean icons that are scalable and can be manipulated (colours, shadows, and - do not use this - reflections).