Every top level record type also has a MAIN PICTURE that can be set. For projects, this main picture is used as part of the Project Portfolio. The Main Picture button is located in the Main Toolbar of each record type. To set the main picture, click the Main Picture Button and then on the browse button in the resulting pop-out window to browse your files & Apply.
You can also just copy an image (right click + copy) and paste an image directly by using the "Paste Image" icon in the top right corner.
See the View Portfolio section for additional information on how to setup Project Portfolios.