Well, actually..... If you are using BlueZX3's RRExtended plugin, you can do this very easily.
Put this in your exectbl.ini
(This line gets the last 3 characters from the song path whenever the song playing changes, thus getting the file type, and saving it in the variable SongIndType )
Then in your skin, put this line:
(This example line will put the image in the top center of the screen, 25 pixels wide, and 25 pixels tall. change the 371,8 coordinates to where you want the image to show up in your skin)
In the I01 line above, change the path to where you want to save the indicator images that you want to use as the indicators. Name the images with the extension of the audio file. For example, for MP3's, the indicator image name should be mp3.png
Thats it! When the song changes, it will update the extension, and thus the image displayed for your indicator.