Fatal Error by BT Slider

Hi there,
I purchased a time ago your Joomla template Galiana through envato.
No I've got a problem with the BT Contentslider Module.
I delivers fatal errors and I don't know why.

Error Message:
Fatal error: Call to undefined function imagecreatefromjpeg() in /var/www/03/00/15/gasthaus-hirschenwirt.at/www/modules/mod_bt_contentslider/classes/images.php on line 120
So I out commented the mentioned lines. Beside line 120 it was line 121, 131 and 140.
After that the whole website works again. More or less (some troubles with content...)

Please let me know what the problem caused and how i can fix it. Out commenting whole code lines can't be a satisfying solution any one of us....
Thx.
Christoph

Hi Christoph
Sorry for late reply. we tested on the site we do not see the problem as you say. please send me an account the site back-end via message private. I'll check for you in the shortest time. Thanks