the code for an image button would be:
<input type="image" name="Submit_order" id="Submit_order" border="0" class="eC_FormButton" src="path/to/image.jpg" />
be aware, however, that changing to an image input type may break server behaviors on the page if they use the submit button for a trigger.
with a regular Submit button type, there is a value attribute passed by the browser that can be used to trigger a server behavior. the trigger code for a submit button type would look like:
with an image input type though, there is no value posted to use for triggering a behavior, instead it passes the horizontal (x) and vertical (y) coordinates of where the mouse click the image. the trigger code for an image button would look like: