Ringo plays .wav audio and shows .jpg images, so most files need converting before you copy them across.
Music: .wav at a 44100 Hz sample rate, which is the default in most converters.
Images: .jpg at 160×128 pixels, so they line up properly on the screen.
To set the sample rate, use an online converter such as this one, which lets you choose the rate before you download the file. For pictures, this converter will turn your image into a .jpg.
Transferring the finished files to Ringo is covered in the Ringo build guide on our Resources page.