Much of the asset creation process involves manipulation of images, textures and photos, so you need a professional-level image editor. GIMP is an open source graphics editor which in the past was based mostly on Adobe Photoshop. That means if you've done graphics work before in Photoshop, you should be able to recognize most of the functions in GIMP. The reason we're using GIMP in this book is that not everyone has access to Photoshop, which is a costly piece of software, and lower cost versions such as Photoshop Elements just don't have the functions required to do asset creation. If you have Photoshop installed, you can use that instead without too much difficulty, as Photoshop has all the functions of GIMP and more.
Note
To install GIMP go to http://www.gimp.org and click on the download link. The Windows installer should be at the top of the list. For the MAC version click on show other downloads and scroll down to GIMP for Mac OS X.
Now it's...