This is a limitation of HTTP and not GD. The reason is that you can only output one type of content^[1] per request e.g either the content is an image or the content is text.

An option here would be to have <img> tags whose src attribute points to a perl script that prints out the requested image. So your content-type will be text/html, while the images your perl script will output will be image/png.

For more info on HTTP check out the rfc.
HTH

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broquaint

^[1] you can have multiple types of content (see. here), but that won't do what you expect.