It should be selected already but if not Command-Click it once to select it
Go to the File menu and select Import (Command-e)
Choose the image you want to import. Note: if the image has a higher resolution than 72ppi it will appear smaller in the item than the original. This happens because Freeway converts images to 72ppi (the web standard) by default. Resizing the image and then resampling (by double-clicking on the graphic and clicking "Resample") will bring it back to its original quality. You can also check the Ignore Resolution checkbox in the Import dialog when you import the graphic
Click Open
Go to Item>Graphic to see a number of options to fit your graphic to the item (or the item to the graphic). You can also Control-click on the item (or right-click with a two-button mouse) to display the contextual menu where the same options are available in the Graphic sub-menu.
Page 112 of the Freeway Reference guide goes into more detail about graphics and images in Freeway.