Captcha are not shown in comments

Hi, I just upgrdated the SI Captcha plugin on my website (http://lucacerone.net if you want to check).

Unfortunately the plugins doesn’t work now.

Depending on the browser I get two different behaviours:

Using Chromium the Captcha doesn’t appear at all.
Using Firefox the Captcha box is shown, as well as the background and a few lines, but the numbers and letters are not shown.

Is there an easy fix for this???

Thanks in advance for the help, and for developing this useful plugin!

http://wordpress.org/extend/plugins/si-captcha-for-wordpress/