problems with Html URI validator

Dear W3C Team,
my name is Eugenio Boscolo and I'm an Italian Web developer.
I have a problem with your HTML Uri validator.

I'm trying to validate a web page by Uri and the validator reports the
following error:
http://validator.w3.org/check?uri=http%3A%2F%2Fwww.mps.it&charset=%28detect+automatically%29&doctype=Inline&group=0

The error text is:
Line 41, Column 154: there is no attribute "border".
…i.gif" alt="Ricerca nel sito" border="0" />

But in my rendered page there is not such code ('border="0").
You can verify it by browsing at http://www.mps.it and viewing source.

As a further evidence, if I try to validate my page by direct input, the
online validator reports the following message:
'This Page Is Valid XHTML 1.0 Transitional!'
Can you plese tell me if I am missing something, or if it's just a
validator's bug?
Thank you in advance.
Best regards,
Eugenio Boscolo