Gimp is a very good program like Devynex said^^
I suggest looking at this >for help regarding map maker. (http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=1391.0 this tutorial[/url)
Adding to Dev, last time I checked you could only use one layer. I don't think you can have 3 and them merge them all together.