Image fit modes can be changed from the tab context menu:
Or from a button at the playback panel:
Image fit modes are the following:
Keep the image view scaling the same as the original image, and show the whole image at the view area.
Try to keep the image view scaling the same as the original image, fill all available view areas and cut part of the image off if needed.
Change image scaling to stretch the whole image in the available view area.
The image fit button is visible in the playback panel if it is enabled in the playback panel controls settings: