what do I use for src attribute on a form image input

I'm using a third-party form, so I have limited flexibility on the html. One thing I want to do is use an image input

<input type="image" alt="Sign up >" src="???" />

in place of their submit input. I have a button image attached to the page containing the html form. My question is what do I use for the src attribute on the input? I tried using the url when I hover over the attachment name (i.e. download/attachments/6651930/sign_up_button.png), but results in a broken link.