Basically what you want to do is get the filename (with the relative path i.e. ../myfile.jpg) that you want to delete, then check to see if it actually is a file with is_file(), then unlink it if is_file() returned true.
$filename = "../myFile.jpg";
if(is_file($filename)){
unlink($filename);
}
This probably isn't the best example, but if you could post some code I could probably help you out more.