Do you like the smiling bioharzard symbol in the address bar? This is often called a favicon, short for “favorites icon” because when you save a link to your bookmarks or favorites, the favicon shows. WordPress.com calls it a Blavatar for “Blog Avatar” but refers to it on the dashboard as Blog Picture / Icon. You can have one too.
On the Dashboard, go to Settings (at the bottom of the left column), click on General. Follow the directions to upload a .jpg or .png file. Don’t bother trying to upload an .ico file or a .gif file. You can upload a normal Web-sized image and WordPress will prompt you to crop the image to a square.
Wait a few minutes for it to percolate through the system. For more information, see this.