"Signature Verification Failed" when flashing stock ROM When booting into recovery mode and installing a ROM zip from my SDcard, it first says I need to mount my /sdcard (when it is mounted), and then it says "signature verification failed", despite my using a stock ROM zip.

Perhaps this has something to do with my obtaining the stock(?) ROM from a possibly untrustworthy third-party site? What is the official download site for Samsung Galaxy S4 (SGH-M919) phones?

thanks in advance for the help

(cf. my other question for more background information)
I have found sammobile.com a very useful site for finding firmware for samsung devices. I don't think there is an official download site.
Here is the firmware for SGH-M919

I have had the same issue before. Just trying the same process again fixed it for me.
You can try wiping your cache and dalvik cache but I am not sure if that will make a difference. Then retry flashing the ROM.
Also make sure you have downloaded the file correctly. Trying to Install a partially downloaded file will give you a lot of headaches!