How to disable cache, image headers?

Hi, I'm creating images on the fly. They are called as follows: <img src='mypicture.php' ...> Where mypicture.php creates a png file, modified each time. Everything was fine with IE, but then I tested it with Netscape and it didn't work anymore. An old version of the picture was shown on the screen. I understand NS is using a homemade cache system, so I tried the usual "Pragma: no-cache" header, but it didn't seem to work either... Any idea how I can force NS to update the image? Thanks, Arnaud PS: I don't want to use a trick like adding a random number at the end of the picture's url, it's for a professional application...