Hi,
I've check the image, both the thumbnail and the original and the border issue comes from the thumbnail generation process, not the CSS or HTML of the email.
The issue with image scale down processes is that they don't handle well stripes. If you have stripes or 1 pixels like that:
on an image and that you resize it to be half as high, you'll either get only black, or only white and no stripes anymore.
That's the same process you're experiencing here.
The solution is either to have bigger thumbnails or to modify the original image to reduce the "stripe" effect by having a thicker border.
When you edit your emails via the menu System>Emails, you have a preload area where you'll find that line of code:
$img = $imageHelper->getThumbnail($item->images[0]->file_path, array(50, 50), array('forcesize' => true, 'scale' => 'outside'));
There, the 50,50 defines the size of the thumbnails in the email. If you increase them, you'll be able to have bigger thumbnails, which hopefully will avoid the problem. So try several values there until you have the border displayed.