Codeigniter "file_exists($filename)"
Codeigniter is always running on index.php, so all paths are relative from there. You can use any of the following, assuming upload/ is at the same level as index.php:
file_exists("upload/$id.jpg")file_exists("./upload/$id.jpg")file_exists(FCPATH."upload/$id.jpg")
FCPATH is a constant that Codeigniter sets which contains the absolute path to your index.php.
As a side note, I prefer is_file() when checking files, as file_exists() returns true on directories. In addition, you might want to see if getimagesize() returns FALSE to make sure you have an image.