Are ROMs legal if you own the original game

No. You cannot legally download a ROM image even if you own the game. While US copyright law does permit a backup of most media (providing you make the backup yourself), the verdict of the Atari vs. JS&A lawsuit would indicate that video game cartridges are non-volatile and are thus not permitted to be duplicated, even for personal use. Also, if the game has any form of copyright protection on it, copying it would be a violation of the DMCA (Digital Millennium Copyright Act).