you are running your app on your mac, but your image path is a windows path.
ImageIcon i = new ImageIcon("C:/character.png");
. The path on Mac should be "/User/xxx/your_projects/background.jpg". This is an absolute path. You can check the image path by keyboard shortcut "command + i". And you can also use relative path to your project if you have the image file in your project.
ImageIcon i = new ImageIcon("background.jpg");