The Windows 7 SP1 Official ISO images hosted on Digital River are genuine and official version, and the downloads from the server is totally free and definitely legal. It’s after all one of the official channel where end-users who purchase a license (product key) for Windows 7 get the setup installation files.
Anyway, without a genuine and legally purchased product key, you can’t get Windows 7 activated, and Windows 7 will only run as evaluation copy for up to 30 days.
Links to download Windows 7 SP1 Official ISO from Digital River:
Windows 7 Ultimate SP1-U ISO
Windows 7 Professional SP1-U ISO
Windows 7 Professional N SP1-U ISO (Note: N editions come without media components)
Windows 7 Home Premium SP1-U ISO