Hello steffi-02
It may be that the image downloaded is currupted . Cna you download it again and try or check the file checksum matching it with the one in download site .
MD5: ad57c1adad9045a43ac58e6bfc7cfad5
SHA1: 7dd81102c37f66c290abdc5eba10258bfd252a79
To check this of a file : run
> file checksum md5 < image location/name>