I spent some time playing with this today and was able to duplicate the problem easily. The image in mail doesn't change - whichever picture shows when you first look at the email seems to stay with that message no matter how many times you change the image on the server. Even when I forward the message to myself it doesn't reload the picture. Wow. And I couldn't find the images anywhere in any cache, either.
I did find a reference from last summer to this same problem with no real answer. One person agreed with Leandro Ardissone's answer and recommends using OnyX to "trash the Application Cache", but it's only a one time fix.
The "dummy query string" suggested by Ates Goral worked great for me, so if you can deal with changing that every time you change the picture that seems to be your best bet.
Wish I could be more help, but I'm stumped.
I think we need someone from the Apple Mail group to enlighten us to a real fix!