Have a site up but think I need to upload wp-content, not sure where

I had a friend create a site in WP for me awhile ago. Now I’m having the content copied to a new domain on a new host. The header image that was on the original site appears to have been a background image and I found how to remove that from Appearance >Editor. The issue is, I can’t get a new image in it’s place.

I asked the original creator how to and they said to look in the wp-content folder, but when I try to go there I just get a blank page. Based on other posts, I’ve re-downloaded WP 3.2.1 and see the wp-content folder, but now what? Where/how do I upload it? And once it’s there, how do I get the image I want in it? And more importantly, is this what I need to be doing to get the new url for the background header I want??