so it should be possible for Godot to display the PNG like that, but how? Do I have to change some other options in order to actually use Antialiasing?
Downscaling large images will cause grainy artifacts. In 3D this is usually solved with mipmaps, but it’s best to keep your image resolution accurate to how it will be displayed in-game.
Windows photo viewer uses a better downscaling technique because it’s only goal is to show one image, whereas Godot has to show hundreds or thousands of images 60 times per second.