Tobii Pro Studio does not scale images. It will always show them in the original resolution.
This means that if you want images to be displayed in such a way that they fill out the screen you will need to change their size accordingly. Most likely this would be 1280x1024, or 1920x1200 if using the Tobii ProT60XL, but it would ultimately depend on what resolution you have setup in the Windows Display Settings.
You can use any image or video editor to change the image or video size.