If you use mail like Outlook that by default displays html then you'll see the image ad (filled with the true contents plus links and whatever else they choose to put in it) the spammers intend for you to see, but the part that the filter looks like is the text of nonsense words. If you have your email set to display text and not the html, then you see the nonsense and not the pictures. The spammers are presuming that most people use the html view.

SRS