Odd. I read mail using Thunderbird, which can handle HTML mail well, and the message had lots of other graphics displaying as usual.
Looking at the message 'source code', I see there IS a graphic link, but it's
img alt="" border="0" src="/Images/6388.jpg" />
The URL is missing the root http://www.sqlservercentral.com/ (which the other images do have), so it's looking for 6388.jpg on my
disk instead of on SQLServerCentral's!