If you absolutely must use input, try this:
background-image: url(...);
background-repeat: no-repeat;
background-position: <left|right>;
padding-<left|right>: <width of image>px;
It’s usually a little easier to use a button with an img inside:
<button type="submit"><img> Text</button>
However the browser implementations of button for submitting are inconsistent, as well as the fact that all button values are sent when button is used – which kills the “what button clicked” detection in a multi-submit form.