Change submit button image the very moment when radio button is selected in php

Change submit button image the very moment when radio button is selected
in php

This is the dynamic radio button:
echo "<input type='radio' name='q_id' value='.$ans[$i].'
id='.$i.'>$ans[$i].";
and here is the submit button:
<input type="submit" name="submit" id="submit" value="Submit"
style="border: 0; background: transparent"><img src="images/skip.png"
alt="submit_x" width="223" hspace="40" height="49"/>
I want to change the image source in the button the very moment when radio
button is clicked. New image source will be "images/next.png"
This change in image has to be done before submitting the form.