It is highly recommended to upgrade your ROMMON to at least 15.0(1r)M16 before jumping to the latest 15.7 releases to prevent "invalid image" flags. Fix/Recovery Steps:
This specific M (Maintenance) release addresses known vulnerabilities and stability issues found in earlier 15.7 versions. C1900-universalk9-mz-spa-157-3-m9-bin Fix Download
: Ensure you can ping the TFTP server from the router's command line. It is highly recommended to upgrade your ROMMON