Timeblog.net

This maybe a really serious security threat to Windows. Image files in the Windows MetaFile Format (WMF) can contain code! This can be used to start malicious code by simply looking at a picture! So, do not look at pictures in E-Mails ending in .wmf, these mails WILL come! Turn off any feature of your mail program automatically opening pictures! Make sure the ending is jpg or gif or png or whatever, but NOT wmf before opening a picture!

Further information:
Financial Times Article
F-Secure News