Currently, you cannot directly download a public image. 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 download the private image to your local PC.
Helpful links:
- Creating a System Disk Image from a Windows ECS or Creating a System Disk Image from a Linux ECS
- Exporting an Image
Windows and SUSE Linux public images and the private images created from these public images cannot be exported.