Tap, and hold the image; then choose "copy," open a new browser window, and paste into the address bar (or in TD's case paste the link in the img window.)
Holy shit, I've used iOS since 2008 and I am only figuring this out now.
Until today, if I was on my iPhone and I wanted to post a pic from a web page, I would long press and save the picture to my phone and then upload to imgur app and then use that link to post the image here when making a post.
But there was this easy solution all along. My mind is blown...
Example: Image: http://www.newyorker.com/online/blogs/cartoonists/daily-cartoon-1123-465.jpg