Why Is Downloading A Picture Potentially Harmful?

I've seen numerous references to why one might want to prevent pictures downloading for security reasons. But I've not been able to find descriptions of what bad can happen and/or how it can be bad. I've read emoticons can carry additional malware, etc. Would someone send pointers to technical descriptions of how dowloading/viewing a jpg file can be harmful? Does viewing a picture run code?

you must mean this:

http://www.microsoft.com/technet/security/…n/ms04-028.mspx

http://www.computerworld.com/printthis/200…4,96124,00.html

http://www.aladdin.com/eSafe/JPEG-Exploit.asp
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a second opinion here:

http://vmyths.com/column/1/2004/10/18/

http://www.vmyths.com/hoax.cfm_id=95&page=3.htm