only 9 number which one are you ommiting? 0-9 = ten numbers.😃
Actually you'll need 26 lowercase letters, 26 uppercase letters, 10 numbers and whatever punctuation you're allowing in your username field. So it'll be 62+ graphics.
Okay now that, that's out of the way. It's pretty easy.
Refer to http://www.asciitable.com and name each graphic after it's ascii value. Then in the code just do something like this:
<?php
function str2img($string) {
$retval = '';
for($i=0;$i<strlen($string);$i++)
$retval .= '<img src="images/' . ord($string{$i}) . '.jpg">';
return $retval;
} //end str2img
?>