Looking at the
input element specification, it looks like src is a valid attribute so this should work:
HTML Code:
<input src="/path/to/myImage.png" type="submit" value="Submit" name="submit" />
and you can probably use CSS too if you want (better option IMO).
xhtml:
Code:
<input type="submit" value="Submit" name="submit" id="submit" />
css:
Code:
#submit
{
padding: 10px;
margin: 0;
background: #fff url(/path/to/myImage.png) no-repeat;
}