You can import any BMP, JPG, or GIF image to use as a 2D background. You can also alter them to fit the proportion of your screen, see display mode.
To import am image to use as a 2D background:
Click the Scene button.
Click the 2D Background button then the Modify tag.
Click the Import button in the Import Image field and browse to the location of the image file you want to use.
Choose the file type, select the image file and click the button to import the file.