An error has occurred. 0 Call to undefined function imagecreatefromjpeg()

Hello,
I am trying to get a slideshow on my website. If I use your demopictures, everything works. Whenever I use my own pictures and I try to load the page, there comes an error: An error has occurred. 0 Call to undefined function imagecreatefromjpeg(). If I try a smaller sized picture this error occurs: 0 Call to undefined function imagecreatefrompng(). I dont know anymore.
Can you help me?
Thanks!

Hi
go in the module options >> advanced >> auto create thumbs >> set it to NO
the error shall be removed
CEd

Its now working, thanks! How big is the size of the pictures I can use? I used very small pictures now, but the quality is very low..

And is there a way to use portrait pictures? Or can I only use the same sized landscape pictures?

the image size depends on your own needs, you can use any image
if you want to combine horizontal and vertical images, you can switch the option "adjust image" so that it will not fit the image in the area

Thanks! Where du I find the "adjust image" option?