Spammers often put images into messages to hide the text from the content filter. Images are characterized by the following HTML code: <IMG src=filename>
Since there are no words outside the HTML tag in the above example, you would only see a graphic when you open the message. The statistical filter alone would not decipher this HTML code, because all words are included within the HTML tag. But the HTML parser will decode the HTML to see if it contains any Image tags.
If you want all messages containing IMG SRC tags to be considered spam, select this option under HTML Feature Filtering.
If you want the domain name in such a tag to be considered a spam indicator, place the domain name in the URL Domain Black List.