If you have ever wanted to manipulate images under linux you probably have usedGimp. This isn't your only option and if you want to do things from the command line a better option is to use ImageMagick's convert utility.
I've put together 5 simple command line examples that I have found useful. This is just a sample of what you can do with convert. To see more examples and get more explanation of options see: ImageMagick v6 Examples.
I started with the following image as a base for all the examples that follow: