I want to know is it safe to click on an image on facebook to be displayed in my browser, ( in case it's a real image and not a link to external URL).
I'm asking about facebook especially and all websites generally.
David has a valid point about image-based exploits (we did see a few) - but those are the kind of threats where ordinary folk can't keep losing sleep on.
Mainstream browsers are mostly safe if you only open the image. Do make sure -
that you're not opening the image in an untrusted site's html page. You did mention this - direct image link, not an external html page. Be wary of being redirected from what looks like an image link though.
to protect against script attacks after redirection, use a script-blocker (like NoScript) or ad-blocker (like uBlock Origin and/or uMatrix).