As I wrote you by mail, I implemented your idea in the following way. As you
say, it's only working on Gecko based systems. I did not implement additional
configuration parameters, but made it through the CSS file:

- adding a new CSS style "attachmentframe" around all attachments
- adding class "attachmentframe" to the 'default.css' like:

.attachmentframe {
  border:1px solid grey;
  border-top:1px solid lightgrey;
  border-left:1px solid lightgrey;
  background-color:white;
  padding:5px;
}

.attachmentframe img {
    max-width:100%;
}

The latter restricts the image size to 100%, which is the width of the 
browser window. I like it that, but of course you can write 200px or 
anything else. Please find the modified elogd.c and default.css under CVS.