You cannot directly download a public image. However, you can use the public image to create an ECS, use the ECS to create a private image, export the private image to your OBS bucket, and then download the image.
Windows, SUSE, Red Hat, Ubuntu, and Oracle Linux public images and the private images created from these public images cannot be exported.
However, if a Windows, SUSE, Red Hat, Ubuntu, or Oracle Linux private image is created from an external image file, this private image can be exported.