Good question, not sure I use avast and it catches everything right away, with a big funny nuclear reactor warning sign! If I download I scan first before opening.
Go here and search for "virus" in the page... http://www.faqs.org/faqs/graphics/fileformats-faq/part1/ Answer: not very probable since ET accepts only specific graphic file types, none of which carry, or at least are not known to carry executable code.
gif , jpg, bmp cant have virus's in them, nor can emails. emails with xml java exe, attatchments can though..
I believe some image types (eg bmp) used to be able cause buffer overflows inside old versions IE which allowed viral code to be run. If you are running a unpatched web brower you may be still able to catch a virus from an image file.
That's a problem with the viewing application, not the graphic file itself. And in order to exploit a buffer overrun, some sort of executable code able to explicitly access an exact memory location must execute/run.
A while back some threads were infected with a trojan virus. This happened to me when I was reading a journal thread & Mcafee displayed a message about a trojan virus being blocked. I think it was in the ad which displayed on the thread page. I remember at least twice this happened & people would post a thread named "do not open thread" with the name of the thread. Probably this issue has been resolved.