You can add music to a slideshow by uploading an MP3 file into the slideshow along with the image files. The uploader notes explain that .mp3 files are acceptable for adding to an image slideshow.
After the .mp3 file is uploaded, you will see an "Audio Soundtrack" section has been added to the folder information. To remove the .mp3 file, click the red "Delete Audio Soundtrack" button. Only one soundtrack file can be uploaded to a slideshow at a time.
Once uploaded, the audio soundtrack will begin playing when the first slide starts and continue playing until the audio file ends.
Note: If there are still pictures remaining in the slideshow when the audio ends, the slideshow will continue to advance without the audio until it reaches the last picture - the audio file does not automatically loop. When the slideshow starts over at the first picture, the audio file will begin playing again.
If you want the audio file to loop until the slideshow is complete, you will need to use an audio editing program to create a single .mp3 audio file, looping it multiple times. Similarly, if you want multiple audio files to play during the slideshow, you will need to string them together into one .mp3 audio file using an audio editing program.
Audacity is an easy-to-use audio editing program that can be downloaded for free at http://audacity.sourceforge.net.