Magento: Transactional emails. How add an image from a custom template

Question

I've created in my webpage a custom theme ('mytheme') and I'd like to add to a transactional email an image from this template (.../skin/frontend/mytheme/default/images/myimage.png)

I've tried some options but anyone works:

<div sytle="background: {{skin url="images/myimage.png" _area="frontend"}}"></div>

<div sytle="background: {{skin url="images/myimage.png" _area="frontend" _theme="mytheme"}}"></div>

<div sytle="background: {{skin url="images/myimage.png" _area="frontend" _theme="mytheme" _package="default"}}"></div>

Answer 1

In this case _package would be mytheme

<div sytle="background: {{skin url="images/myimage.png" _area="frontend" _theme="default" _package="mytheme"}}"></div>  

Also verify that the image is there otherwise magento will fallback to the default package