The openclipart Add-on for Google Docs from Apps4Gapps gives ready access to over 50,000 public domain images that you can use in your documents.
The add-on is added in the normal fashion, by using the Add-ons menu item from within a document. To add a piece of art into your document, simply place the cursor where you want the image to appear, then select Add-ons > openclipart > Search Clipart from the top menu:
This opens a sidebar containing the interface to the Add-on:
From here, you can search the image database using keywords:
Finally, to insert your chosen image, simply click on it in the sidebar, and it will be inserted into your document at the cursor position.
Don’t forget that you can then amend the way the image is displayed, an how the text flows around it, as explained in this post.
The available images are a broad collection including book-style drawn illustrations, cartoons, business graphics, icons, notices, speech bubbles and more, so it’s likely you’ll find something suitable for most applications, though the majority are quite light-hearted in nature so may not be so suitable for more formal documents.
This is an independent review based on personal opinion. I have no commercial or personal connection with the authors of the product reviewed.