Transparent filled polygon over image

[SpeedStick]

Hi, I'm working on a map program where each building is clickable and when it's clicked the page is refreshed to show a map with the building highlighted (in red). I was wondering if it would be possible to highlight the buildings using some sort of combination of the imagepolygonfill function and a transparency function (i'd like the buildings to still be visible under the highlight), rather than having to create many different files with the different buildings highlighted. Does anyone know how this could be done? Thanks...

[SpeedStick]

I've managed to solve this problem.... However, in doing so, another one has come up: I'd like to include the image amongst some other PHP/HTML output. However, due the header being set to image/png I am receiving the 'Cannot modify header information' error. Is there anyway to avoid this and include both output and the created image in a file, or do i need to use another method like iframes to embed the image?

Thanks....