First, click the + button and then choose Import. You'll see a toolbar like this:
Clicking on Files tab File manager on your computer. You can navigate to the image/video you want and choose it by double-clicking.
Connect to Drive
Likewise, you can choose the Google Drive tab and access any media you have saved there. You'll need to log in after you click the Drive button.
Once an image or video is chosen, it will be added to the page. You may find it takes a while to load, depending on the size of the file and your internet connection speed.