Hi,
what it does is :
1 - it searches for zenity and for kdialog.. if you have one of them, it chooses it, if you have both then...
2 - it checks the environment variable DESKTOP_SESSION, if it's gnome or kde, it chooses zenity/kdialog depending on that,
3 - if DESKTOP_SESSION is 'xfce' it chooses zenity
4 - it then does a 'ps aux | grep gnome' and 'ps aux | grep kde' and checks the number of processes with 'kde' in their name and with 'gnome' in their name...
5 - if the number of gnome processes is more than the number of kde proceses, it chooses zenity, otherwise, it chooses kdialog.
What I suggest, since noone is maintaining the plugin is that you send us a patch for it to fix it... maybe just add a if DESKTOP_SESSION == fluxbox.. or maybe you could just launch amsn from a script that does :
#!/bin/bash
DESKTOP_SESSION="gnome" /usr/share/amsn/amsn
about the thumbnails, I have no idea, maybe zenity or kdialog have an option for it.. check their man page and adds it to the launch command.