Hosting images on subdomain vs subdirectory and SERPs
 

How bad is it to be hosting images on i.yoursite.com/image.jpg if most people will be linking to that URL and so most of your backlinks will be going to the subdomain? It is said that you are spreading out the link juice on 2 different locations. And if all your backlinks are going to the subdomain then that is what will be ranking high in Google even though there is no site at i.yoursite.com, will yoursite.com never benefit from all the backlinks to i.yoursite.com?

For example Alexa and surely Google realize that i.imgur.com and imgur.com are the same site and pairs them together. But for something like blogpost however they do not:
http://www.alexa.com/siteinfo/i.imgur.com
http://www.alexa.com/siteinfo/imgur.com
http://www.alexa.com/siteinfo/biggho...n.blogspot.com
http://www.alexa.com/siteinfo/blogspot.com

At what point does Alexa and Google begin weighing the subdomain and main site the same and pairing them together? If at all?