Thanks for the reply's. I just found http://khtml2png.sourceforge.net, which is a "a command line program to create screenshots of webpages." (from their site)
However, it requires that the following software is installed on the server:
- KDE 3.x
- kdelibs for KDE 3.x (kdelibs4-dev)
- zlib (zlib1g-dev)
I guess not all of this software is installed, right? Is there a way I could install them? Or does it require an own server / VPS (root access)?