Multisite Subsite Image Path

I hate to ask such a simple question…but, I have set up Multisite in the “directory” (as opposed to the sub-domain) set up and I need to know where I can upload (via ftp) and image file to get a working URL for a sub-account like shown below…

http://mainsite.com/subsite/images/

I’ve tried it in the “blogs.dir” folder and also created a “subsite/images/” in the root installation folder…no avail. I’d greatly appreciate your help!

-Ryan