Since the image shows up in staging we already know that either it changed, or it's new, or it's deleted. All of that information is already available in the UI (the FileViewModel have those properties), so you just need to implement some stuff in the ImageDiffViewModel:
If it's new: display nothing for the old image (no api calls) and display the new image (call /api/diff/image?filename=imageName.png&version=current). You already have the filename in the ui so this should be possible to do.
If it's removed: Do just about the same thing but call (/api/diff/image?filename=iamgeName.png&version=previous) for the old image and display nothing for the new one
If it's changed: A mix of both