To actually make use of the form data, the method depends on whether you want to return the image alone to the browser or whether you want it contained in a proper web page.
If it's the former you can use $_GET or $_POST directly in a GD script as jojoba suggests. If it's the latter you can read the form input, put it in the src of your <img> tag as appropriately urlencode()d GET values, then read it from $_GET in the GD script.
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