"image not found" error in sysupgrade

I'm attempting to force sysupgrade to revert my router back to an OEM firmware. My dual partition router had oem firmware on part 1 and openwrt on part 2. I accidently flashed my first partition to openwrt as well. I've been in a headache trying to revert my first partition today. I'm not too tech savy but this has been my attempt so far. (I'm on mac os)

copied oem firmware to /tmp ssh in, cd to /tmp (success) sysupgrade -n -F linksys.img an error pops up saying image not found.

Any help with this? I have a short time frame to get this done and sent on its way. Any help would be appreciated.