Vertical 1px on image in windows outlook 07' 10' html email

I have a Vertical white 1px on my image in windows outlook 2007 & 2010 for my HTML email. I don't know why this is happening. This only happens in windows outlook so far from what i've seen. And not any other client.

enter image description here

Heres a photo

<tr>
    <td id="header" class="w640" width="640" align="center" bgcolor="#FFFFFF">
        <img editable label='header image' src="images/header.gif" class="header" width="640">
    </td>
</tr>

Solution

I came across two possible reasons for this behavior of borders in Outlook while googling :

Outlook adds 1px border to the table cell elements. You can get rid of it by using border-collapse : collapse CSS property to your table cells and cellpadding="0" and cellspacing="0" attributes to the table element. The strange thing that this border appears to be only on the right side of the td, but it can be so due to <img> layout. If you want to look for more information about this issue you can follow this link.
If you are setting somewhere in CSS classes your border to something like border: 0px style color (or not setting at all) (the main part here is setting border-width to 0px) it will be ignored by Outlook, so you can make it to not display border at all by setting border-style : none. I guess this is closer to your problem, as I've found a similar sample picture here.