Moving code from a specific file to a generic one

Hi, I have this source code in this randomImage.php file (see source): http://www.tuxdoit.com/randomImage.phps This generates a random image for form validation. Now, as you can see, i call the picture from http://www.tuxdoit.com/newslwetter.php as <img src="randomImage.php"> and that works just fine. But... i'd like to include the source code of randomImage.php in a generic functions.php file along with others. My problem is i'm not being able to do that. If i move the code to functions.php, what modifications must be made to the image generation code and how do i call it from newsletter.php ? I've already tried to make it a function, of course, but no good :( I'm driving nuts with this :( Any help would be apreciated. Warm Regards, MARG