I'm pretty sure vB uses GD to generate the image on the fly.
So you wouldnt be able to find it in your images directory, To change the size just jump into your styles and templates editor in your control panel, select the register template under edit tempaltes and look for the line
Code:
<td rowspan="2"><img src="image.php?$session[sessionurl]type=regcheck&imagehash=$imagehash" alt="$vbphrase[registration_image]" width="201" height="61" border="0" /></td>
Just change the width and height sections - I think thats prolly the easiest way.