First off you will need any image editing software that uses layers. This type of software ranges from Gimp to Photoshop. The software I am using is Paint.Net. Note that these softwares need to have Layers options.
List of software that have layers:
-Gimp
-Paint.Net
-Art Weaver
EXPENSIVE SOFTWARE:
-Photoshop
-Corel Painter or Draw
Duration : 0:10:1
[youtube 8652sp1BnRo]