lucy24 - 11:07 pm on Jan 31, 2013 (gmt 0)

"No referer" includes:

99% of all search engines, good or bad
and
some {browsers | hosts | proxies} with or without the human user's knowledge or consent
and
anyone, anywhere, pasting the image URL directly into their address bar, or using a bookmark

So it depends what you mean by "safe". You can choose to make exceptions for known quantities, such as your favorite search engine. But it is well-nigh impossible to block all referer-less requests without preventing some real humans from seeing your pictures.

Rather than [F] I'd rewrite to something like a single-pixel gif. (A "No Hotlinks" or "Stop Thief!" image would be unfair, because some of your affected humans don't know they're not sending a referer.)

Either way, make sure your page code includes explicit height and width declarations for all foreground images, so overall page layout isn't affected.