first check your config.php file in the inc/ folder and see is everything match with the new domain, and if you have also installed the forum, you need to update the database: mybb_settings in there you change the url to the new url domain, then delete the settings.php in the forum/inc/ folder or where you installed the forum. a new setting.php file will be created automatically with the new info.
then you will also need to update the images files you already have, because if you used the upload image from the adimincp the complete url is added to the database the tables you need to look are:
filesmap, levels and items
you can use one of the codes from here:
http://stackoverflow.com/questions/1...-a-mysql-query to update the tables with the new info, for me the first answer always work (the one that says: For a single table update)