TheMadScientist - 3:59 am on Feb 24, 2011 (gmt 0)
I don't usually have anything to do with image rankings, but I can tell you:
1.) If you host on http://example.com/ you will probably need to use a different site to host to get the full speed benefits, because if you use http://cdn.example.com/ cookies from http://example.com/ will be sent and if you're going to move them for search / speed purposes you might as well do it right.
2.) Where ever you decide to host them, make sure you 'set the home directory' of the sud-domain or new site to the current domain and block / redirect access to anything except the images back to the main domain, or 'point' the new site / sub-domain directly to the images location on the current site and adjust your links, because if you move the images physically to the new domain or sub-domain you create a management PAIN IN THE A** (Been there, done that.).
In short: I recommend keeping the images right where they are in the site structure, but changing where they are requested from, rather than physically moving them if at all possible.