Captcha not showing image correctly when set to use GD support

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I modified the code to use GD support. The example file displays the image fine. But my Concrete5 form does not.

What do I need to change to get Concrete5 captcha to work in GD?

Example form:
http://www.conf.gnsi.org/concrete/libraries/3rdparty/securimage/exa...

My Concrete5 form:
http://www.conf.gnsi.org/2013-contact-us/...

Thanks,
Jerome