Need help with new plugin

[Been Told]

Hi there,
here is what I want my plugin to do:
The user chooses their country of residence in their CP (Custom Profile Field).
Then in postbit_legacy I take the name of the country the user chose and make an image out of it.
So a bit like this:

PHP Code:

$flag = $_GLOBALS["field11_for_example"];
$flag2 = str_to_lower($flag);
$flag2 = trim($flag2);
$flag = "<img src='images/flags/".$flag2.".gif' border='0' alt='".$flag."' />";
$_GLOBALS["field11_for_example"] = $flag; 


Now I know I can't access the profile field the way I do in the above code. But what is the right way?
I basically just want to assign field11 a new value.
Can anyone help me with this?

And what hook do I use? I thought it may be showpost_start or postdata_start ???

Thanks in advance for any help! :up: