Just a small update:
The image preview worked great on my live server but it still didn't display the images in the filebrowser on my local testserver.
I run a win7 64bit machine.
The solution to that is very simple, I had installed wamp server 64 bit, once I installed wamp server 32 bit every thing worked fine on my local testserver.
I hope this can be helpfull for other people if they encounter the same problem