It'll be those images with, "look what you've been caught doing" messages. The user opens the image and BINGO!
Microsoft is investigating private reports of a vulnerability in the Microsoft Graphics component that affects Microsoft Windows, Microsoft Office, and Microsoft Lync. Microsoft is aware of targeted attacks that attempt to exploit this vulnerability in Microsoft Office products.
The vulnerability is a remote code execution vulnerability that exists in the way affected components handle specially crafted TIFF images. An attacker could exploit this vulnerability by convincing a user to preview or open a specially crafted email message, open a specially crafted file, or browse specially crafted web content. An attacker who successfully exploited the vulnerability could gain the same user rights as the current user. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.Microsoft Warns of Vulnerability From Hacked TIFF Images [technet.microsoft.com]
Microsoft is offering a temporary Fix-It for this issue: https://support.microsoft.com/kb/2896666 which, if I understand it correctly, will prevent you from viewing TIFF files. Expect a better fix from Microsoft in their next round of patches.