Paint.NET and GIMP
You will work with images at some point during Android development. Most professionals use Adobe Photoshop, but you may not be able to shell out much money for an image editing program. Therefore, you have two free alternatives: Paint.NET and GIMP.
The Paint.NET image manipulation program was written on top of the .NET Framework. Paint.NET, which is targeted for Windows, works great and is used by many developers around the world.
The GIMP open source program is similar to Photoshop. GIMP can be installed on Windows, Linux, or Mac.