WordPress links images to internal ip

Hey!
I have just installed wordpress on my webserver.
I run ubuntu 10.04.1
Apache version 2.2.14
The server is behind a router with the external adress of: lusborgen.mine.nu

Now I already have a gallery3 in my /var/www folder
I have put the new site “kastanj” in /var/www/kastanj

All works fine and dandy on my gallery3 site but my wordpress site seems angry. It shows its anger when browsing from outside my internal network. It then tries to access pictures (maybe other things too) from the internal ip: 192.168.1.3

Im using webmin to administer everything as Im not very skilled in ubuntu. Have searched the web for a solution and found more users having the same issues, seems to be something with the hosts file. However I have a problem understanding the descriptions, can you explain it to me in easy steps I would be greatful!

Also the “kastanj” site will have a new external adress once my brother (his bands page) buys the domain, how do I prepare for that?

Great big thank you if you can help me out!