Imgur does something goofy with the referer that you've clicked a link from. I'm by no means a competent web programmer or understand how linking/referrers/etc. work but at one point I had a little doodad in GreaseMonkey/Tampermonkey that pulled imgur images in and plopped them on your Facebook page.

What I found was that if the page referrer was Facebook, you got nothing, the imgur site attempted to redirect you back through to their galleries, but if you used the link directly you'd get the image.

I never figured out exactly what's happening, I used a code snippet that stripped out the referrer info and everything started working.