Is there a way to make an image a download once you click on it (without right-click save image as)?

I'm using a small Javascript function to call the download page:

<a href=# 

   onclick=window.open('download.php?file=test.jpg', 'download', 'status=0');

>Click to download</a>

In the download.php page I have something like:

$file = $_GET['file'];

header('Content-Description: File Transfer');

header(Content-type: image/jpg);

header(Content-disposition: attachment; filename= .$file.);

readfile($file);

But it doesn't work. What am I doing wrong?

Thanks in advance!