I cropped that Milwaukee file to 1025x1025 and it worked fine. I used an old copy of GraphicConverter, though really any image editing program should be fine. SC4 seemed to be fine with .jpg files but didn't like a .png.
You can save yourself a lot of typing by creating a shortcut from the root of your hard drive into the ".../Documents/SimCity 4" folder. A normal finder shortcut will not work, but you can use the Terminal to create a symlink (which is a different sort of shortcut).
1. Start the Terminal (in Applications/Utilities).
2. type "cd /", no quotes, and hit return.
3. type "ln -s " (no quotes, don't hit return)
4. Open the relevant Document/SimCity 4 folder in the finder. Drag that folder onto the terminal window. This will paste the folder's path into the window.
5. Finish the command by typing " sc4" and hitting return.
6. Test the command typ typing "ls sc4" and hitting return. This should list the contents of your sc4 documents folder.
This will set up a link from /sc4 that points into the simcity documents area. If you put an image called mymap.jpg in the ".../Documents/SimCity 4" folder then you can just type /sc4/mymap.jpg in file dialog. Also, if you open a finder window for your Macintosh HD you should see an sc4 shortcut that brings you to the SC4 documents folder.
