Both music and image files have to be in a specific format in order to work.Music files have to be in .wav format and image files in .jpg format. In case you wish to put your own music files, the sample rate needed is 44100Hz, which should be standard for most .wav file converters.
To convert your wave files to the proper sample rate there are hundreds of sites like these where you can set your desired sample rate when downloading.
For your own images, convert them to .jpg by using a site like this.
Note that the image size must be 160×128 pixels in order to get the proper alignment on the screen.
On how to transfer these types of files to Ringo, check out our little tutorial.