You will be working with images at some point in your Android development career. Most professionals use Adobe Photoshop, but not all of us can shell out that much money for an image-editing program. Therefore, you have two free alternatives: Paint.NET and GIMP.
Paint.NET is a free image-manipulation program written on top of the .NET Framework. Paint.NET works great and is used by many developers around the world. This application is targeted for Windows. Get Paint.NET here: www.getpaint.net.
The GIMP application is an open-source program that is similar to Photoshop. GIMP can be installed on Windows, Linux, or the Mac. See www.gimp.org.