I cannot stress enough the importance of reading the Image Functions section of the PHP Manual, at http://www.php.net/manual/en/ref.image.php. This chapter has given you only a brief introduction to the basics of creating and manipulating images with PHP, and in no way has it taught you everything there is to know-far from it!
As with almost everything in programming, understanding the process is the first step in applying the methods. You've learned that you must have a drawing canvas-be it a blank canvas or a canvas consisting of an existing image. You've also learned that you must allocate colors to the image, and that all things image-related come in steps. Study the processes and do a lot of trial-and-error with creating things like pie charts, bar graphs, resized images, and so forth. That's really the best way to learn image creation-by practicing the small parts.
Additionally, you may find it interesting and useful to read the nuts and bolts information about the underlying libraries, such as the GD Graphics library and the JPEG and PNG libraries. Also, keep up on the browser support for image functions; for example, PNGs are not supported by all browsers, but are used to create much better images than GIFs and even JPEGs, in some instances. Such information is especially useful when determining the types of graphics you'll use PHP to create for your intended audience.