It’s not hard to find images to use online – just use an image search. This works well, but it’s worth noting that doing so isn’t necessarily legal. Most of the images you can find this way are owned by their original creators. This likely doesn’t matter if you’re only using something for private use, or even a school assignment, but if you intend to publish a work you need to make sure all rights are cleared.
Is there a potential downside? Yes. Just because a search engine sees something as Creative Commons doesn’t necessarily mean that it is. Here at MakeUseOf, for example, we’ve had at least one photographer threaten to sue us over an image he didn’t realize he himself licensed as Creative Content. He backed down when we pointed this out, and it’s one example resulting from thousands of blog posts spread over a decade, but know that this isn’t without risk.
If you mean your own artwork, then you'd take a picture of it, upload it to your computer (make sure you know where you're saving it), then select Insert > Picture in Word, browse in the location where you saved the photo, click the picture, and insert. If you mean an image from the internet or something, you can just save it to your computer and basically follow the rest of the steps above.
While most references to Clip Art disappeared with Office 2013, users were able to insert the old-school images into documents using an Office.com Clip Art option. That is now being replaced by Bing Images, with Microsoft filtering images to ensure they’re based on the Creative Commons licensing system for personal or commercial use. Most of the new images are much more modern, instead of the illustrated remnants of the past. Clip Art might be facing the same Office-related demise as the great Clippy assistant, but let the images below remind you of the good old times before the modern-era takeover.