I need a way to upload an image to use as a background for my container. Then, once it is there, I need to know where it is (the path) to use it as a background.

You can upload it into the "disk":
more commands / Files on disk

Then you can upload it to the pics folder by mouse's context menu (right button) or by drag dropping it with Firefox:
http://www.editsite.net/news/drag_and_drop_from_desktop.html

The pic's url wil be /pics/yourpic.gif