Captcha image fail

Dear Sirs,

The captcha image is failing to show… what could be?

But it´s not for all the website, just some pages, please take a look:

1- http://www.obrasdarte.com/historia-da-arte/ – OK

2- http://www.obrasdarte.com/pintura-34-exposicao-virtual-de-artes-visuais-da-indiana-preeti-thaker-arora/ – FAIL

Regards,

Tony

http://ww.wp.xz.cn/plugins/cbnet-really-simple-captcha-comments/