Here are some instructions on how to successfully flash a new Android System Image.
First download and extract the Android System Image that you want to flash. I'll be using hammerhead-mob30h, which is the May security update for my Nexus 5 (2013).
Extract the archive and enter the directory. You'll also want to extract the image-*.zip file as well (image-hammerhead-mob30h.zip in my case), which contains the other img files.
cd ~/Downloads/image-hammerhead-mob30h