Pixabay Images is a WordPress plugin that let's you pick CC0 (Creative Commons Zero) public domain pictures from Pixabay and insert them with just a click anywhere on your blog. The images are safe to use, and paying attribution or linking back to the source is not required. The plugin lets you choose photos, clipart, or both. It also lets you search for horizontal or vertical images.
The plugin will put a new button next to your Add Media button (if you choose that in the settings). Clicking on it will let you search Pixabay and choose the image size.
Once you choose an image there, you are taken to another screen that will let you review and change things such as the title, caption, link, size, and alignment.
If you want public domain images of very good quality, this plugin is a great choice.