Hi,
From what I can see on your link, the error is:
Warning: getimagesize(/www/htdocs/w011ce31/webseitennachmass/hekotextil_new/media/com_hikashop/upload/20120718163154-532f0356-me.jpg): failed to open stream: No such file or directory in /www/htdocs/w011ce31/webseitennachmass/hekotextil_new/administrator/components/com_hikashop/helpers/image.php on line 184
So one easy thing to solve the problem temporarily is to turn off the "error reporting" of the Joomla configuration.
That way, the error message won't appear and it should help the uploader working properly.
Now regarding why the image size couldn't be retrieved, please check for that file in that folder. Check that the permissions are correct, etc.
We'll look at adding a patch on our end for the next version to avoid the warning message.