I was looking for something else and stumbled across this page.
It has a php snippet to download a pdf and I have just tried it with a jpg for both firefox and ie, and it worked okay.
/* Using PHP to force a file download
by Chuck Joslin - Awave, Inc.
This short script forces a file download.
For simplicity, it's intended to be used for a single file.
You can use multiple copies of this file (with unique names)
with different variable values to use it with multiple files.
Use of this script has a side-effect of hiding the location of the
file on your server.
// full server path to file to be downloaded (including filename)
$Path2File = "/home/username/example.com/test/w.jpg";
// the filename
$theFileName = "w.jpg";
//the work gets done here
header ("Cache-Control: must-revalidate, post-check=0, pre-check=0");
header ("Content-Type: application/octet-stream");
header ("Content-Length: " . filesize($Path2File));
header ("Content-Disposition: attachment; filename=$theFileName");
Adjust $Path2File and $theFileName as required and save and run with a name say 'force.php' or whatever.
I am sure it could be modified to add a number of files.
I do not know if it works on a Mac though.