You can change the display method of the image by setting the 'Internal Display Mode'.
By default, images that you have added yourself or copied from a website are entered with 'Original Proportion' as the 'Internal Display Mode'. You can change this setting to 'Fill Screen' and then use the handles to adjust to the desired size.
Furthermore, when the 'Internal Display Mode' is set to 'Fill Screen' or 'Slowly Enlarge', you can easily fit the entire screen by using the 'Auto Fit' function on the right side panel.