From the Symantec Security Response:
http://securityresponse.symantec.com/avcenter/attack_sigs/s21196.html
"ICC Profile data may possibly be embedded in various file formats, including JPEG, GIF, EXIF, TIFF, PNG, PICT, PDF, PostScript, SVG, JDF, and CSS3. Some of these formats may not provide an attack vector, especially if Microsoft does not provide native support or does not call the vulnerable functionality when handling certain formats. Formats that may not be affected due to lack of native support are PDF, PICT, and PostScript, though this has not been confirmed.
Successful exploitation may result in execution of arbitrary code in the context of the currently logged in user. This vulnerability could be exploited through a Web site that hosts a malicious document, by previewing or opening malicious content in email, or through other means that will allow an attacker to send the victim a malicious document."
Could it be that one of the pictures from all those adds has embedded ICC profile data?
BTW, this vulnerability was discovered on July 12. Symantec added it to their definition files shortly after that. Walt initial post was on July 16.