This tutorial describes how to scan emails for image spam with FuzzyOCR. FuzzyOCR is a plugin for SpamAssassin which is aimed at unsolicited bulk mail containing images as the main content carrier.
Read it at howtoforge
Using different methods, it analyzes the content and properties of images to distinguish between normal mails (ham) and spam mails. FuzzyOCR tries to keep the system load low by scanning only mails that have not already been categorized as spam by SpamAssassin, thus avoiding unnecessary work.
I do not issue any guarantee that this will work for you!