An e-mail advertisement in the form of an image in the message rather than text in order to avoid detection as spam. Spam filters typically analyze words in a message, which is relatively fast, but scanning images with optical character recognition (OCR) to extract the text is slow. In addition, spammers have already circumvented optical scanning by fracturing the text within the image similar to the CAPTCHAs on Web page forms (see CAPTCHA). Animated GIFs are also used to spread the text across multiple frames, making it harder to analyze.

Message Fingerprinting
Another defense is to track known image spam and save the attributes of their text and images as signatures that can be quickly analyzed. In many cases, the fingerprint need only be of the image. Although this is a faster detection approach, keeping up with the latest image spam signatures is like keeping virus signatures up-to-date. See spam and OCR.