With a display_callback for your fields you can do what you want.
You can even use the same callback for several fields - but they have to be of the same format.
For instance
public static function display_checkbox($record, $nolink, $desc) {
$field_id = $desc['id'];
if ($nolink) { // do not create img - return raw value
return $record[$field_id];
} else {
$filename = $record[$field_id]; // use value as filename without extension
$f = Base_ThemeCommon::get_template_file('Custom_MyModule', $filename . ".png");
if (!$f) $f = Base_ThemeCommon::get_template_file('Custom_MyModule', "default.png");
return "<img src=\"$f\">";
}
}
