I'll add 2 + 2 for you:
The link provided in my first post shows how to load the image and how to use it for an ImageView.
Now that you can reference any drawable from the SD card, you have to define your structure.
Example: all my skins are stored in /sdcard/myapp/skins
each file must have a file name starting with myapp_skin
Now that that's out of the way, in our program, we reference /sdcard/myapp/skin directory and grab a list of all files stored there. Then based on what we get, we put those files as the drawable resources.
You get each of your images as a File, then generate an InputStream from that file. Finally, you can use either BitmapFactory or Drawable to create the Bitmap/Drawable, whichever you're more comfortable with.