To make SC4 load your file you need to put it anywhere inside the SC4 sandbox folder.
How I did it: (if you have a file called my.jpg and your mac username is well "username" - if not change to whatever is appropriate)
1. Do the usual preparations for regions as before (found in ~/Library/Containers/com.aspyr.simcity4.appstore/Data/Documents/SimCity 4/Regions/)
2. Copy my.jpg file to ~/Library/Containers/com.aspyr.simcity4.appstore/Data/Documents/SimCity 4/
3. Open SimCity4 and open your region
4. When you have your blank region ready press Shift+Ctrl+Alt+R
5. In the file name box enter (by hand) your full path to your file /Users/username/Library/Containers/com.aspyr.simcity4.appstore/Data/Documents/SimCity 4/my.jpg
6. (Curse that you can't copy and paste because you wrote one letter wrong, and in that case nothing happens and you need to do it again)
7. Voila, magic happens
Tips! To find your Home Library folder (a.k.a ~/Library) go to a Finder window, Go menu and hold your Alt/Option key and you will see it (maybe localized into your local mac language)
PS. The reason why it is not possible to see anything is because SimCity tries to load files from the root folder / of your computer and is denied access as it is sandboxed and have no root access, this will fail also attempt to create new folder. It should be an easy option for Aspyr to make it point it directly to folder inside the sandbox structure instead.