Ok, if you are REALLY sure that this is the right path, remove this code from the top of "admin/thumbnailer.php" and "admin/resizer.php" and try if it works.
if ($config['convert_tool'] == "im" && !is_executable($config['im_convert_path'])) {
echo "<b class=\"marktext\">".$lang['im_error']."</b><br />\n";
echo $lang['check_module_settings'];
show_admin_footer();
exit;
}
Jan