You need to send out the appropriate HTTP header in your image generation CGI script, which should be at minimum:

#!/usr/bin/perl -w
print "Content-Type: image/png\n\n";
print "../images/pic1.png";
[download]

Update ...and as others have pointed out, you need to print the file itself to STDOUT, not just the name of the file.

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important