hello n00bster,
Sorry for late reply to this,
Thanks for take your time to described on your current issues,
I have checked in your JomSocial stream, it seems like the first 2 image is not found from your server, that was the reason why the image is how broken now.
-> JoomlaFolder/images/blog/images/64/news/2013/adventkalender-uebersicht-intro/2e1ax_silverfuchs_frontpage_adventkalender-2013-intro.jpg
-> JoomlaFolder/images/blog/images/64/news/2013/adventkalender-uebersicht-intro/2e1ax_default_frontpage_adventkalender-2013-intro.jpg
However, I have tested in my locally and publish a test blog post in your site, it seems like working fine for me even i changed to another template.
If you wouldn't like this default behaviour, you can modify the code under this file :
JoomlaFolder\components\com_easyblog\classes\image.php
//LINE 305
$fileName = $prefix . '_' . $this->activeTheme . '_' . $size . '_' . $this->image;
//Replace with
$fileName = $prefix . '_' . $size . '_' . $this->image;
Hope this help.