Normally yes, although in this case the other inputs have a different width, which interferes with the submit button width.
Ah, missed that. Since I can't style selects etc, I am so used to styling input[type=text] and leaving IE6 alone. So my submits don't usually get hit.
Since this is an image, I would actually go ahead and set a width, and a height. I usually do if I'm doing it that way.
I should also mention the HTML is invalid. Forms are one of those special blocks who cannot hold inlnes (or inline-blocks) as direct children. Usually a fieldset fixes this problem, but since that generally requires a label (differs from HTML to XHTML) some people prefer a div.