>I've tried to follow Tore's suggestion, checking the Internet Cache, used his procedures, etc... but could not find the Captcha image from that site...
>
>If you have any alternative idea, please let me know.

What's in the page's source? Is the image given as an IMG tag pointing to an actual file, or is it given as a base64 inline data string? In the latter case, you can strextract() the string from the page and strtofile() it into a jpg file.