Thanks, this worked. As you suggested, I used the CGI script itself as the image. Passing an argument of 1..4 in order to tell the script what image I wanted.

<img src="/cgi-bin/trend.cgi?image=1">
<img src="/cgi-bin/trend.cgi?image=2">
<img src="/cgi-bin/trend.cgi?image=3">
<img src="/cgi-bin/trend.cgi?image=4">
[download]

The CGI script, or course, accepted the param and returned the corresponding image.