the map has to be stored in directory specified in the options in the format of:
{dir name}/{zoom level}/{x}/{y}.jpg

I think it also expects maps to be split up into 256x256 squares. I was not able to find a definitive howto or anything, just what the author has on the projects home page.

if coordinates are not important to you, you can try putting you jpeg in 15/0/0.jpg and see what it does. worste case mapper will crash