There's the share button at the bottom, but that will share the URL of the page itself that you're looking at.
If you're sourcing from imgur (can't speak to other image hosting sites), it's trivial to then alter the URL to just the image instead of the webpage.
Zaap, long pressing on an image lets you copy it to your device, but unless the place you're posting it will also host it... It's kludgy, but I suppose you could copy it, re-host it on imgur (or similar, I just browse imgur a lot), and then use that image URL.