[Lynne]

Try disabling your modifications/plugins and see if you still have this problem.
Note: To temporarily disable the plugin system, edit config.php and add this line right under <?php

PHP Code:

define('DISABLE_HOOKS', true); 


Don't forget that for vbseo, disabling the modification means to also get rid of anything extra you added to your htaccess file.

If it's still happening, reupload all the original vB non-image files (except install.php). Make sure you upload these in ASCII format and overwrite the ones on the server.