To change the image you will need some sort of image editing software.

Commercial Applications

PhotoShop

This is one of the more powerful software application available on the market.
This is paid for software, however there is a 30 trial available on their site.
http://www.photoshop.com/

Free Software Options:

1. PhotoScape

2. GIMP for Windows

3. Paint.NET

4. Serif PhotoPlus

5. Photo Pos Pro

6. Pixia

Online Editor (recommended)

http://pixlr.com/editor/