Try changing the httpport to 80 in mtaserver.conf:
<httpport>80</httpport>
Restart the server, and try the internet browser again using the new port:
http://XXXXXXXX:80/admin/client/images/warning.png
If that still doesn't work, please tell me about the server IP:
Is the server IP you are using from the LAN i.e 198.168.xx.xx
Or is it the external IP (i.e. the one displayed at http://whatismyip.com) ?