Tobii Pro Studio does not scale images. It will always show them in the original resolution.
If you want images or videos to be displayed in fullscreen you will need to change their resolution according to the resolution of the screen (which is set in the Windows Display Settings).
Most likely this would be 1280x1024 if using the Tobii Pro T60/T120 or 1920x1200 if using the Tobii Pro T60XL/TX300. Always check the resolution of the screen you use in the Windows Display Settings before editing your images and videos.
You can use any image or video editor to change the image or video resolution.