Imagecache pciture wont show in mail template

Hey there,

I got a little problem with the use of imagecache additional to simplenews.
When I'm trying to add a simplenews preset to a pic in my mail template it wont show up in the mail. When I am using the same code in the node template, it works without any problems.
This is what my code looks like:

<?php print theme('imagecache', 'bild_hauptthema', $field_pic_hauptthema['0']['filepath']); ?>

That's what the HTML Code shows in the node:

<img src="XXX/newsletter/sites/default/files/imagecache/bild_hauptthema/newsletter_bilder/D_300x300px_2-1.jpg" alt="" title="" class="imagecache imagecache-bild_hauptthema" width="320" height="320">

And that's what it looks like in the mail:

<img src="http://redaktion-pb.hessen.de/newsletter/sites/default/files/imagecache/bild_hauptthema/" alt="" title="" class="imagecache imagecache-bild_hauptthema">

So as you can see, the filepath part isnt included?!?
Anyone got a idea how to fix that?

Greetings

SG