You can't do this....
What you need to do is associate the image with a URL so it gets generated when the browser processes the <img> tag.. UNLESS it's very small in which case you can use the src="..." attribute to contain the data.
So a PIL image should be returned by a script or external method that you can browse to. Headers such as Content-Type should be set to set the image type.
Instead of generating the image on the call to your ZPT you form an <img src="/some/url/that/makes/my/image?foo=bar&baz=bat"> e.g. in order to generate the right image.
Hi Andrew thank you for your answer. I tried this but calling the page template only shows the image and the other content is not shown. I guess, it has something to do with the REQUEST.RESPONSE.setHeader('Content-Type', WEB_FORMAT) I set in the external method. Here is the code I use: External method "createThumbnail": ---------------------------------- def createThumbnail(self, photo): """ create PIL image """ <snip> thumb_data = StringIO() # 'thumb' is a PIL.Image.Image instance thumb.save(thumb_data, 'JPEG') REQUEST = self.REQUEST REQUEST.RESPONSE.setHeader('Content-Type', 'JPEG') return REQUEST.RESPONSE.write(thumb_data.getvalue()) Page Template "thumbnails.html": -------------------------------- <h1>Thumbnails</h1> <p>some other content...</p> <tal:rep tal:repeat="photo photos"> <a tal:attributes="href photo/absolute_url;"> <img tal:attributes="src python: container.createThumbnail(photo)" /> </a> </tal:rep> <p>some other content...</p> In the page templates I iterate over a list of image objects ("photos") and pass a single image object to external method. As I mentioned above, this only shows the first image and not the other content nor the other images. Did I miss something? Regards Nico