Image Spam

Image spam is junk email that replaces text with images as a means of fooling spam filters. 

Image delivery works by embedding code in an HTML message that links to an image file on the Web. Image spam is a larger drain on network resources than text spam because image files are larger than ASCII character strings. 
Larger files require more bandwidth and, as a consequence, cause greater degradation of transfer rates. 
If the recipient's email program downloads images automatically, the image appears when the message is opened. The image itself may be a picture or drawing of alphanumeric characters that appears as text to the viewer, although it is processed as an image by the user's computer. Many spam filters, especially older or less sophisticated ones, rely upon certain text criteria on which to make judgments. 

Such filters typically watch for predetermined words in the subject lines of e-mail messages, suspicious word patterns and word frequency. Image spam is not stopped by such filters because it contains no words that can serve as the basis for blocking messages. 

There are a number of approaches to dealing with image spam. In current versions of Outlook, for example, images are not automatically downloaded into messages unless the user has specified that messages from the source can be trusted.