Hi andreas.
I've had no luck solving this. But the way I did it the tell-a-friend
email works properly - complete with product thumbnail.
The emails without images are the order email and wishlist email.
A few days ago walteis posted a topic with something simular, with
the difference that he managed to have a wishlist email which 'only'
misses the first image. The rest of the products al have images!
You can find it here:
http://forum.x-cart.com/viewtopic.php?t=16841
No idea what causes this and it's a pity that nobody else seems to
know it either.