Pixabay is one of the best places to get quality images for free. This website adopts a license similar to Creative Commons Zero (CC0), allowing you to use its images for any purpose without crediting the author. If you often use Google Docs to create a document, Pixabay can be a great resource to get the supporting images for your document.
Instead of downloading the image first to re-upload it later to add it to the document you are working on, you can directly add an image from Pixabay to Google Docs. There is an add-on (Pixabay Free Images) you can make use of. To install the add-on, go Add-ons -> Get add-ons.

Type “Pixabay” on the search box and click the FREE button to install it on your Google Docs.

Once installed, you will have a new item on the Add-ons menu on your Google Docs. To add an image from Pixabay, put your cursor within your document where you want to insert the image to and go to Add-ons -> Pixabay Free Images -> Search Images.

A panel will appear on the right side of the screen. Type the image keyword on the search box on this panel to search an image and hit enter. Simply click on the image to add it to your document.

Final words
The Pixabay Free Images add-on is pretty helpful if you often use Google Docs and always use images every time you create a new document. One thing, it is not an official add-on from Pixabay. Instead, it is developed by a third-party developer who makes use of the Pixabay API to provide the Pixabay image library on Google Docs. You can visit the developer website here.