Hi,
I have exact the same Problem http://doksoft.com/forum/?Itemid=126&catid=4&id=12&option=com_kunena&view=topic. Upload works fine. But activate resize produce a HTTP Error (500).
There is no error in the Error Log.
Are there any Systems requirements? (PHP Version, ImageMagick,...) ?

Thanks
Marco

Hello,
Yes, you need to install GD. Usually name of the package is: php5-gd
Do not forget to restart a web server after installing it.

awesome! works like a charm!