how to block an imagen code

hey i'm having trouble signing by book because i can't see the image code

this is what appears when i click send

"Guestbook - Error
One of the input fields does not seem to be valid."

i'm pretty sure that what i'm missing is the image code that is not displaying on my site which is

this "http://sergiohmunoz.com/guestbook/index.php"

please help me i need to unable the image code so people don't have to put that... but how?????

Your image code is displaying fine here. I even signed your guestbook with no problems.