Searching for and inserting Clip Art or pictures from other online sources from within PowerPoint isn't currently supported in PowerPoint 2016 for Mac. However, you can still add Clip Art and online pictures to your PowerPoint presentation by searching for the photo you want online, saving a local copy of it, and then inserting the copy you saved into PowerPoint. Here's how:
Microsoft Office online website is a great resource for those who want to learn about Microsoft products and download assets and resources like free clipart for PowerPoint and other Microsoft Office products.
LibreOffice (formerly OpenOffice) are useful free and open source office productivity softwares. You may download them for free now, and start using on most major platforms.