[Zope] question on howto readfile(img)

i have what i hope is a fairly easy question that's been alluding me for a few hours. i've been trying to rewrite a little script i wrote in php long ago. i need to have a script, which returns an image once it decides it exists... the only thing i don't have worked out is how to do something like this: <dtml-var RESPONSE.setHeader('Content-Type','image/jpeg')> readfile(img+?size=thumb); <<----not sure how to do this, this is from php img being a path given through the url: ie ?img=somefolder/someimage.jpg thanks, holden