If you are using the new SpamFilter 4.0, you may safely disable (or make less agressive) the scanning of images within emails and PDFs, as in this new release there are several new filters which render the image filter obsolete (especially if you have enabled the greylisting filter). The image filtering can be disabled by going to the "Settings - Configuration tab", then clicking on the "Filter Settings" button. On the window that follows, please enter a value of "0" for the Image filter threshold (a screenshot for this setting is available at http://www.logsat.com/sfi-spam-filter-screenshots/sfi-filtering-options.asp).

You can also disable the filter that scans for spam signatures only within PDF files by modifying the following parameter in the SpamFilter.ini file with this value of zero:

SpamPDFMaxPagesToScan=0