How to add an image to asciidoc book cover page

I would like to create a pdf book using asciidoc. The title page should include the title, subtitle and an image. So far I could not find any documentation on how to achive this.

Intuivly I would do something like

My book title
=============
:author: myself
:toc:
image:images/titelimage.png[width=400]

but this just adds image:images/titelimage.png[width=400] as a subtitle. What I would like to have is something like

My book title
<titleimage>
subtitle

Is there a way to do this in ascciidoc? If not, what do I need to do to get this?