If you have access to a Flickr album, and it has the appropriate permissions (is public, and is attributed for reuse) you will be able to insert media from that Image URL:
Step 1: Get to the Flickr album
Step 2: Choose your photo by clicking on it
Step 3: Use the share buttons
Step 4: Copy the “share” URL
Step 5: Paste this URL into the image URL spot.
Note: If the preview image does not appear, you probably do not have the correct URL.