lxml question

[mattia]

I would like to click on an image in a web page that I retrieve using 
urllib in order to trigger an event. 
Here is the piece of code with the image that I want to click:
<input type="image" style="border-width: 0px; height: 22px; width: 49px;" 
onclick="return checkPhoneField(document.contactFrm, 'mobile');" 
alt="sms" src="images/button_sms.bmp" id="smsToMobile" name="smsToMobile"/
>

I don't know how to do it (I'm trying using lxml, but any suggestion can 
help).

Thanks, 
Mattia