Hi!
Sadly this is an issue with the Internet Explorer. It is unable to parse transparent PNG images, and thus we created the plugin "browser compatibility" which converts the transparent images to be used by IE.
However, IE is unable to properly display PNG icons the first load when no width/height is given in the HTML source.
So, there are three solutions to the problem:
1. Browse the serendipity code and add width/height to all HTML syntax for <img> elements. This however will put a fixed width on icons for any theme, so different themes could not use smaller/larger icons. Thus, this route is not really good.
2. Remove the browsercompatibility plugin and view the non-transparent PNG images.
3. Use a decent browser like Firefox (or wait for IE7).
Sorry for that hassle

( But I'm glad to hear you like SErendipity!
Best regards,
Garvin