I recently upgraded to WordPress 2.5 and things seemed to go fine, until I tried uploading an image to a post. There are two bugs in the code which cause problems. I was able to find a simple fix that works. You’ll need to edit two files in your WordPress install:

wp-admin/includes/media.php
     Line 817 (debug: false) : Remove the trailing comma

wp-includes/js/swfupload/handlers.js
     Line 99 (height:26) : Remove the trailing comma