Bit.ly allows you to view stats for short URLs by attaching a + sign at the end of the URL.
Example: http://bit.ly/1dAtI4q
To look up Stats: http://bit.ly/1dAtI4q+
My question is, if I include this URL in an image src attribute like this:
<img src="http://bit.ly/1dAtI4q">
and embed this image on a page and request the page, the stats from bit.ly do not count this request as a click.
They do however count visiting the URL via the browser by pasting the URL in the address bar of the browser.
Question: How do they distinguish between the URL is embedded in an image SRC and someone is visiting the URL?
Bitly can't actually distinguish between when a bitly link is followed to fetch an image src, and when a user really clicks on the link, at least not at the point that it records per-link statistics. Your browser cached the redirect after you first followed the link; that's why the image src didn't record another hit.
Try looking at this page: http://www.ploxiln.net/test.html
and looking up the stats for the image hot-linked through a bitly link: http://bitly.net/GFcvn1+
Now refresh the test page, and refresh the stats. You might need to force a "full" refresh with Ctrl-Shift-R, on the test page.
(Disclaimer - bitly engineer here :)