The Image Filter is a Plugin. That means it is not directly integrated in Spamihilator, but it can be used to block Spam mails.
This filter searches mails for images that are saved on external servers. Spammers often use this method because they want to keep the size of their mails small.
It is also possible to get information about your mail address! When you open such a spam mail, the spammers will know that your mail address exists! Thus, it's better to block these mails before they get into your mail client!
Mails that contain images as an attachment will not be blocked by this filter. Use the Attachment Filter instead.