Force refresh of graphic - how?

I have a PHP page which in turn calls an image-creating PHP program, like so: ...(calling page: mypage.php).. <body> <IMG SRC="makepic.php?opt1=abc&opt2=def"> .... </body> </html> makepic.php does database lookups using the values of opt1 and opt2, then creates a graphic (PNG) using that data and returns the graphic. So far, so good... but the graphic is being cached, so the data is not always up-to-date, unless I do a shift-reload of the calling page. I tried adding a "pragma nocache" line to the HEAD of mypage.php, but that had no effect. I don't want to tell my users to always do a shift-reload - how can I force either mypage.php to not cache the graphic, or get makepic.php's output to not be cached???? Thanks... replies to wilson@cpuworks.com