Hello

I have installed and enabled the Plugin on Joomla 1.5.22
The dir for Images is images/stories.
I have type into an Article,
{gallery}Kreisübung_2012_014.jpg{/gallery}

Failure:

Warning: opendir(/www/htdocs/w00b56d8/images/stories/<i><i>Kreisübung_2012_014.jpg</i></i>) [function.opendir]: failed to open dir: No such file or directory in /www/htdocs/w00b56d8/plugins/content/jwsig.php on line 100

Please read SIG documentation.
The tags you use are wrong.
You should use a file for your gallery not just an image.