If you are using non-standard Image Path setting for custom styles, please make sure you upload that file to the appropriate 'editor' for your custom style.
Upload the sprite.png file to /forum/images/styles/yourstyle/editor....
Code:
images/editor/sprite.png
That was lifted from a similar question on the other vbulletin site when I was looking for fix. Dunno if it will help you or not, but there it is.